user27295: im trying to connect wifi module to my MEGA 2560 board.
i set up the board as the picture bellow

and my program is as follows 
#include <SoftwareSerial.h>

#define DEBUG true

SoftwareSerial esp8266(18,19); // make RX Arduino line is pin 2, make TX Arduino line is pin 3.
                             // This means that you need to connect the TX line from the esp to the Arduino's pin 2
                             // and the RX line from the esp to the Arduino's pin 3
void setup()
{
  Serial.begin(9600);
  esp8266.begin(9600); // your esp's baud rate might be different

  pinMode(11,OUTPUT);
  digitalWrite(11,LOW);

  pinMode(12,OUTPUT);
  digitalWrite(12,LOW);

  pinMode(13,OUTPUT);
  digitalWrite(13,LOW);

  pinMode(10,OUTPUT);
  digitalWrite(10,LOW);

  sendCommand("AT+RST\r\n",2000,DEBUG); // reset module
  sendCommand("AT+CWMODE=1\r\n",1000,DEBUG); // configure as access point
  sendCommand("AT+CWJAP=\"mySSID\",\"myPassword\"\r\n",3000,DEBUG);
  delay(10000);
  sendCommand("AT+CIFSR\r\n",1000,DEBUG); // get ip address
  sendCommand("AT+CIPMUX=1\r\n",1000,DEBUG); // configure for multiple connections
  sendCommand("AT+CIPSERVER=1,80\r\n",1000,DEBUG); // turn on server on port 80

  Serial.println("Server Ready");
}

void loop()
{
  if(esp8266.available()) // check if the esp is sending a message 
  {


    if(esp8266.find("+IPD,"))
    {
     delay(1000); // wait for the serial buffer to fill up (read all the serial data)
     // get the connection id so that we can then disconnect
     int connectionId = esp8266.read()-48; // subtract 48 because the read() function returns 
                                           // the ASCII decimal value and 0 (the first decimal number) starts at 48

     esp8266.find("pin="); // advance cursor to "pin="

     int pinNumber = (esp8266.read()-48); // get first number i.e. if the pin 13 then the 1st number is 1
     int secondNumber = (esp8266.read()-48);
     if(secondNumber>=0 && secondNumber<=9)
     {
      pinNumber*=10;
      pinNumber +=secondNumber; // get second number, i.e. if the pin number is 13 then the 2nd number is 3, then add to the first number
     }

     digitalWrite(pinNumber, !digitalRead(pinNumber)); // toggle pin    

     // build string that is send back to device that is requesting pin toggle
     String content;
     content = "Pin ";
     content += pinNumber;
     content += " is ";

     if(digitalRead(pinNumber))
     {
       content += "ON";
     }
     else
     {
       content += "OFF";
     }

     sendHTTPResponse(connectionId,content);

     // make close command
     String closeCommand = "AT+CIPCLOSE="; 
     closeCommand+=connectionId; // append connection id
     closeCommand+="\r\n";

     sendCommand(closeCommand,1000,DEBUG); // close connection
    }
  }
}

/*
* Name: sendData
* Description: Function used to send data to ESP8266.
* Params: command - the data/command to send; timeout - the time to wait for a response; debug - print to Serial window?(true = yes, false = no)
* Returns: The response from the esp8266 (if there is a reponse)
*/
String sendData(String command, const int timeout, boolean debug)
{
    String response = "";

    int dataSize = command.length();
    char data[dataSize];
    command.toCharArray(data,dataSize);

    esp8266.write(data,dataSize); // send the read character to the esp8266
    if(debug)
    {
      Serial.println("\r\n====== HTTP Response From Arduino ======");
      Serial.write(data,dataSize);
      Serial.println("\r\n========================================");
    }

    long int time = millis();

    while( (time+timeout) > millis())
    {
      while(esp8266.available())
      {

        // The esp has data so display its output to the serial window 
        char c = esp8266.read(); // read the next character.
        response+=c;
      }  
    }

    if(debug)
    {
      Serial.print(response);
    }

    return response;
}

/*
* Name: sendHTTPResponse
* Description: Function that sends HTTP 200, HTML UTF-8 response
*/
void sendHTTPResponse(int connectionId, String content)
{

     // build HTTP response
     String httpResponse;
     String httpHeader;
     // HTTP Header
     httpHeader = "HTTP/1.1 200 OK\r\nContent-Type: text/html; charset=UTF-8\r\n"; 
     httpHeader += "Content-Length: ";
     httpHeader += content.length();
     httpHeader += "\r\n";
     httpHeader +="Connection: close\r\n\r\n";
     httpResponse = httpHeader + content + " "; // There is a bug in this code: the last character of "content" is not sent, I cheated by adding this extra space
     sendCIPData(connectionId,httpResponse);
}

/*
* Name: sendCIPDATA
* Description: sends a CIPSEND=<connectionId>,<data> command
*
*/
void sendCIPData(int connectionId, String data)
{
   String cipSend = "AT+CIPSEND=";
   cipSend += connectionId;
   cipSend += ",";
   cipSend +=data.length();
   cipSend +="\r\n";
   sendCommand(cipSend,1000,DEBUG);
   sendData(data,1000,DEBUG);
}

/*
* Name: sendCommand
* Description: Function used to send data to ESP8266.
* Params: command - the data/command to send; timeout - the time to wait for a response; debug - print to Serial window?(true = yes, false = no)
* Returns: The response from the esp8266 (if there is a reponse)
*/
String sendCommand(String command, const int timeout, boolean debug)
{
    String response = "";

    esp8266.print(command); // send the read character to the esp8266

    long int time = millis();

    while( (time+timeout) > millis())
    {
      while(esp8266.available())
      {

        // The esp has data so display its output to the serial window 
        char c = esp8266.read(); // read the next character.
        response+=c;
      }  
    }

    if(debug)
    {
      Serial.print(response);
    }

    return response;
}

but i always recieve the following error 
Arduino: 1.6.12 (Windows 7), Board: "Generic ESP8266 Module, 80 MHz, 40MHz, DIO, 115200, 512K (64K SPIFFS), ck, Disabled, None"

Sketch uses 228,949 bytes (52%) of program storage space. Maximum is 434,160 bytes.
Global variables use 32,796 bytes (40%) of dynamic memory, leaving 49,124 bytes for local variables. Maximum is 81,920 bytes.
warning: espcomm_sync failed
error: espcomm_open failed
error: espcomm_upload_mem failed
error: espcomm_upload_mem failed

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.

any idea where i went wrong ?

user4143: 
Do not use SoftwareSerial on a hardware serial port. It is pointless. 
Do use a voltage divider to convert the Arduino's 5V TX to the required 3.3V the ESP8266 needs.
Do not try and program the Arduino as if it were an ESP8266 - it won't work.

I think you are getting a little confused here...

27295: <p>im trying to connect wifi module to my MEGA 2560 board.
i set up the board as the picture bellow</p>

<p><a href="https://i.stack.imgur.com/b43IC.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/b43IC.jpg" alt="board setup"></a></p>

<p>and my program is as follows </p>

<pre><code>#include &lt;SoftwareSerial.h&gt;

#define DEBUG true

SoftwareSerial esp8266(18,19); // make RX Arduino line is pin 2, make TX Arduino line is pin 3.
                             // This means that you need to connect the TX line from the esp to the Arduino's pin 2
                             // and the RX line from the esp to the Arduino's pin 3
void setup()
{
  Serial.begin(9600);
  esp8266.begin(9600); // your esp's baud rate might be different

  pinMode(11,OUTPUT);
  digitalWrite(11,LOW);

  pinMode(12,OUTPUT);
  digitalWrite(12,LOW);

  pinMode(13,OUTPUT);
  digitalWrite(13,LOW);

  pinMode(10,OUTPUT);
  digitalWrite(10,LOW);

  sendCommand("AT+RST\r\n",2000,DEBUG); // reset module
  sendCommand("AT+CWMODE=1\r\n",1000,DEBUG); // configure as access point
  sendCommand("AT+CWJAP=\"mySSID\",\"myPassword\"\r\n",3000,DEBUG);
  delay(10000);
  sendCommand("AT+CIFSR\r\n",1000,DEBUG); // get ip address
  sendCommand("AT+CIPMUX=1\r\n",1000,DEBUG); // configure for multiple connections
  sendCommand("AT+CIPSERVER=1,80\r\n",1000,DEBUG); // turn on server on port 80

  Serial.println("Server Ready");
}

void loop()
{
  if(esp8266.available()) // check if the esp is sending a message 
  {


    if(esp8266.find("+IPD,"))
    {
     delay(1000); // wait for the serial buffer to fill up (read all the serial data)
     // get the connection id so that we can then disconnect
     int connectionId = esp8266.read()-48; // subtract 48 because the read() function returns 
                                           // the ASCII decimal value and 0 (the first decimal number) starts at 48

     esp8266.find("pin="); // advance cursor to "pin="

     int pinNumber = (esp8266.read()-48); // get first number i.e. if the pin 13 then the 1st number is 1
     int secondNumber = (esp8266.read()-48);
     if(secondNumber&gt;=0 &amp;&amp; secondNumber&lt;=9)
     {
      pinNumber*=10;
      pinNumber +=secondNumber; // get second number, i.e. if the pin number is 13 then the 2nd number is 3, then add to the first number
     }

     digitalWrite(pinNumber, !digitalRead(pinNumber)); // toggle pin    

     // build string that is send back to device that is requesting pin toggle
     String content;
     content = "Pin ";
     content += pinNumber;
     content += " is ";

     if(digitalRead(pinNumber))
     {
       content += "ON";
     }
     else
     {
       content += "OFF";
     }

     sendHTTPResponse(connectionId,content);

     // make close command
     String closeCommand = "AT+CIPCLOSE="; 
     closeCommand+=connectionId; // append connection id
     closeCommand+="\r\n";

     sendCommand(closeCommand,1000,DEBUG); // close connection
    }
  }
}

/*
* Name: sendData
* Description: Function used to send data to ESP8266.
* Params: command - the data/command to send; timeout - the time to wait for a response; debug - print to Serial window?(true = yes, false = no)
* Returns: The response from the esp8266 (if there is a reponse)
*/
String sendData(String command, const int timeout, boolean debug)
{
    String response = "";

    int dataSize = command.length();
    char data[dataSize];
    command.toCharArray(data,dataSize);

    esp8266.write(data,dataSize); // send the read character to the esp8266
    if(debug)
    {
      Serial.println("\r\n====== HTTP Response From Arduino ======");
      Serial.write(data,dataSize);
      Serial.println("\r\n========================================");
    }

    long int time = millis();

    while( (time+timeout) &gt; millis())
    {
      while(esp8266.available())
      {

        // The esp has data so display its output to the serial window 
        char c = esp8266.read(); // read the next character.
        response+=c;
      }  
    }

    if(debug)
    {
      Serial.print(response);
    }

    return response;
}

/*
* Name: sendHTTPResponse
* Description: Function that sends HTTP 200, HTML UTF-8 response
*/
void sendHTTPResponse(int connectionId, String content)
{

     // build HTTP response
     String httpResponse;
     String httpHeader;
     // HTTP Header
     httpHeader = "HTTP/1.1 200 OK\r\nContent-Type: text/html; charset=UTF-8\r\n"; 
     httpHeader += "Content-Length: ";
     httpHeader += content.length();
     httpHeader += "\r\n";
     httpHeader +="Connection: close\r\n\r\n";
     httpResponse = httpHeader + content + " "; // There is a bug in this code: the last character of "content" is not sent, I cheated by adding this extra space
     sendCIPData(connectionId,httpResponse);
}

/*
* Name: sendCIPDATA
* Description: sends a CIPSEND=&lt;connectionId&gt;,&lt;data&gt; command
*
*/
void sendCIPData(int connectionId, String data)
{
   String cipSend = "AT+CIPSEND=";
   cipSend += connectionId;
   cipSend += ",";
   cipSend +=data.length();
   cipSend +="\r\n";
   sendCommand(cipSend,1000,DEBUG);
   sendData(data,1000,DEBUG);
}

/*
* Name: sendCommand
* Description: Function used to send data to ESP8266.
* Params: command - the data/command to send; timeout - the time to wait for a response; debug - print to Serial window?(true = yes, false = no)
* Returns: The response from the esp8266 (if there is a reponse)
*/
String sendCommand(String command, const int timeout, boolean debug)
{
    String response = "";

    esp8266.print(command); // send the read character to the esp8266

    long int time = millis();

    while( (time+timeout) &gt; millis())
    {
      while(esp8266.available())
      {

        // The esp has data so display its output to the serial window 
        char c = esp8266.read(); // read the next character.
        response+=c;
      }  
    }

    if(debug)
    {
      Serial.print(response);
    }

    return response;
}
</code></pre>

<p>but i always recieve the following error </p>

<pre><code>Arduino: 1.6.12 (Windows 7), Board: "Generic ESP8266 Module, 80 MHz, 40MHz, DIO, 115200, 512K (64K SPIFFS), ck, Disabled, None"

Sketch uses 228,949 bytes (52%) of program storage space. Maximum is 434,160 bytes.
Global variables use 32,796 bytes (40%) of dynamic memory, leaving 49,124 bytes for local variables. Maximum is 81,920 bytes.
warning: espcomm_sync failed
error: espcomm_open failed
error: espcomm_upload_mem failed
error: espcomm_upload_mem failed

This report would have more information with
"Show verbose output during compilation"
option enabled in File -&gt; Preferences.
</code></pre>

<p>any idea where i went wrong ?</p>
 4143: <ul>
<li>Do not use SoftwareSerial on a hardware serial port. It is pointless. </li>
<li>Do use a voltage divider to convert the Arduino's 5V TX to the required 3.3V the ESP8266 needs.</li>
<li>Do not try and program the Arduino as if it were an ESP8266 - it won't work.</li>
</ul>

<p>I think you are getting a little confused here...</p>
