user42909: code is compiling, what I want is that when rfid tag kept led should get on but instead its getting to the else loop.
int count = 0;
char input[12]; // rfid input
int led = 3; // led pin
char first[20] = {"270059204B15"}; //rfid tag no
char a[24];


void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(led, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  if (Serial.available() > 0) // code for rfid input
  { count = 0;
    while (Serial.available() && count < 12)
    {
      input[count] = Serial.read();
      count++;
      delay(10);
    }
    Serial.print(input);
  }

  if (strcmp(input, first) == 0) /*comparing two string rfid tag no and input
   if matches led is on*/
  {
    digitalWrite(led, LOW); /* as the board which we are using is in sink
    mode low is taken as 1 or on*/
    delay(500);
    Serial.print("led is on\n");
  }

  else
  { digitalWrite(led, HIGH);
    delay(500);
    Serial.print("led is off\n");
  }
}


user-1: Your unespecified problems are consecuence of a collection of small details.

You are not putting a binary zero to end the input string.
That's not the way to read serial input.
Code doesn't filter out control caracteres (LF, CR).

Here is your code cleaned up:
#define ID_SIZE 12

int count=0;
char input[ID_SIZE + 1]; // rfid input
int led=3; // led pin
char first[ID_SIZE + 1]= {"270059204B15"}; //rfid tag no

void setup()
{
    // put your setup code here, to run once:
    Serial.begin(9600);
    pinMode(led,OUTPUT);
}

void loop()
{

    // put your main code here, to run repeatedly:
    while(Serial.available() > 0 && count < 12) {  // code for rfid input
       char c = Serial.read();
       if (isAlphaNumeric(c)) {
          input[count++]=c;
          }
      }

    input[count] = 0;
    if(count >= 12) {
        Serial.print(input);
    }

    if(strcmp(input,first)==0) /*comparing two string rfid tag no and input
   if matches led is on*/
    {
        digitalWrite(led,LOW); /* as the board which we are using is in sink
    mode low is taken as 1 or on*/
        delay(500);
        Serial.println("led is on\n");
        input[0] = count = 0;
    }

    else {
        digitalWrite(led,HIGH);
        delay(500);
        Serial.println("led is off\n");
    }
}

And here is the result:

Edit:
The C strings are char arrays of undefined lenght. To signal the end of the string, put a binary zero next to the (effective) last element. Then, if you want to store a 10 chars string, you need an 11 elements arrays (10 for the string itself, 1 for the binary zero). 
Failure to put a binary zero at the end of a string is a very common cause of program failure.
Control characters. In general, when you type "TEST" in the Serial Monitor and press SEND, what Arduino receive is the word TEST itself, plus a Line Feed/Carriage Return pair (LF/CR) to mark the end of the line.
In real world, your serial input can be contaminated with tabs, spaces, LF/CR and other valid chars. You need to discard them (unless they are valid for your app).

42909: <p>code is compiling, what I want is that when rfid tag kept led should get on but instead its getting to the else loop.</p>

<pre><code>int count = 0;
char input[12]; // rfid input
int led = 3; // led pin
char first[20] = {"270059204B15"}; //rfid tag no
char a[24];


void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(led, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  if (Serial.available() &gt; 0) // code for rfid input
  { count = 0;
    while (Serial.available() &amp;&amp; count &lt; 12)
    {
      input[count] = Serial.read();
      count++;
      delay(10);
    }
    Serial.print(input);
  }

  if (strcmp(input, first) == 0) /*comparing two string rfid tag no and input
   if matches led is on*/
  {
    digitalWrite(led, LOW); /* as the board which we are using is in sink
    mode low is taken as 1 or on*/
    delay(500);
    Serial.print("led is on\n");
  }

  else
  { digitalWrite(led, HIGH);
    delay(500);
    Serial.print("led is off\n");
  }
}
</code></pre>
 -1: <p>Your unespecified problems are consecuence of a collection of small details.</p>

<ol>
<li>You are not putting a binary zero to end the input string.</li>
<li>That's not the way to read serial input.</li>
<li>Code doesn't filter out control caracteres (LF, CR).</li>
</ol>

<p>Here is your code cleaned up:</p>

<pre><code>#define ID_SIZE 12

int count=0;
char input[ID_SIZE + 1]; // rfid input
int led=3; // led pin
char first[ID_SIZE + 1]= {"270059204B15"}; //rfid tag no

void setup()
{
    // put your setup code here, to run once:
    Serial.begin(9600);
    pinMode(led,OUTPUT);
}

void loop()
{

    // put your main code here, to run repeatedly:
    while(Serial.available() &gt; 0 &amp;&amp; count &lt; 12) {  // code for rfid input
       char c = Serial.read();
       if (isAlphaNumeric(c)) {
          input[count++]=c;
          }
      }

    input[count] = 0;
    if(count &gt;= 12) {
        Serial.print(input);
    }

    if(strcmp(input,first)==0) /*comparing two string rfid tag no and input
   if matches led is on*/
    {
        digitalWrite(led,LOW); /* as the board which we are using is in sink
    mode low is taken as 1 or on*/
        delay(500);
        Serial.println("led is on\n");
        input[0] = count = 0;
    }

    else {
        digitalWrite(led,HIGH);
        delay(500);
        Serial.println("led is off\n");
    }
}
</code></pre>

<p>And here is the result:</p>

<p><a href="https://i.stack.imgur.com/CzJcQ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CzJcQ.png" alt="enter image description here"></a></p>

<p>Edit:</p>

<p>The C strings are char arrays of undefined lenght. To signal the end of the string, put a binary zero next to the (effective) last element. Then, if you want to store a 10 chars string, you need an 11 elements arrays (10 for the string itself, 1 for the binary zero). </p>

<p>Failure to put a binary zero at the end of a string is a very common cause of program failure.</p>

<p>Control characters. In general, when you type "TEST" in the Serial Monitor and press SEND, what Arduino receive is the word TEST itself, plus a Line Feed/Carriage Return pair (LF/CR) to mark the end of the line.</p>

<p>In real world, your serial input can be <em>contaminated</em> with tabs, spaces, LF/CR and other valid chars. You need to discard them (unless they are valid for your app).</p>
