user12160: I’m sorry if I sound ignorant and naive, but I’m brand new to Arduino;  very keen, but know very little.
I started with a Sainsmart Arduino Uno clone board.  I downloaded the software, which opened OK (or at least seemed to), but the Serial Monitor kept telling me that there was a communication error, and a yellow light kept flashing on the Sainsmart board.
I got hold of a genuine Arduino (from Jameco), uninstalled everything Arduino, and started again from scratch.
Now I get the message:
Arduino: 1.6.5 (Mac OS X), Board: "Arduino/Genuino Uno"
Error opening serial port '/dev/cu.Bluetooth-Modem'. (Port busy)
  This report would have more information with
  "Show verbose output during compilation"
  enabled in File > Preferences.

And after enabling the verbose output, got:
Arduino: 1.6.5 (Mac OS X), Board: "Arduino/Genuino Uno"
Error opening serial port '/dev/cu.Bluetooth-Modem'. (Port busy)

Tried again, and:
Arduino: 1.6.5 (Mac OS X), Board: "Arduino/Genuino Uno"
Error opening serial port '/dev/cu.Bluetooth-Modem'. (Port busy)

I’m using a MacBook Pro, using Yosemite 10.10.5.

user11729: You are not selecting the correct port to upload to - hence the strange message about the Bluetooth modem. Start by unplugging your Uno and running Arduino. Select Tools -> Port from the menu bar. You should see a list of options. Now plug in your Uno and do the same again. You should see an extra port listed now - this will be the Uno. Select it. You will need to check this every time you connect a board. 
Also, before you click Upload, make sure you have Arduino Uno selected under Tools -> Board in the menu bar. 

12160: <p>I’m sorry if I sound ignorant and naive, but I’m brand new to Arduino;  very keen, but know very little.</p>

<p>I started with a Sainsmart Arduino Uno clone board.  I downloaded the software, which opened OK (or at least seemed to), but the Serial Monitor kept telling me that there was a communication error, and a yellow light kept flashing on the Sainsmart board.</p>

<p>I got hold of a genuine Arduino (from Jameco), uninstalled everything Arduino, and started again from scratch.</p>

<p>Now I get the message:</p>

<pre><code>Arduino: 1.6.5 (Mac OS X), Board: "Arduino/Genuino Uno"
Error opening serial port '/dev/cu.Bluetooth-Modem'. (Port busy)
  This report would have more information with
  "Show verbose output during compilation"
  enabled in File &gt; Preferences.
</code></pre>

<p>And after enabling the verbose output, got:</p>

<pre><code>Arduino: 1.6.5 (Mac OS X), Board: "Arduino/Genuino Uno"
Error opening serial port '/dev/cu.Bluetooth-Modem'. (Port busy)
</code></pre>

<p>Tried again, and:</p>

<pre><code>Arduino: 1.6.5 (Mac OS X), Board: "Arduino/Genuino Uno"
Error opening serial port '/dev/cu.Bluetooth-Modem'. (Port busy)
</code></pre>

<p>I’m using a MacBook Pro, using Yosemite 10.10.5.</p>
 11729: <p>You are not selecting the correct port to upload to - hence the strange message about the Bluetooth modem. Start by unplugging your Uno and running Arduino. Select Tools -> Port from the menu bar. You should see a list of options. Now plug in your Uno and do the same again. You should see an extra port listed now - this will be the Uno. Select it. You will need to check this every time you connect a board. </p>

<p>Also, before you click Upload, make sure you have Arduino Uno selected under Tools -> Board in the menu bar. </p>
