Error opening serial port '/dev/cu.Bluetooth-Modem'. (Port busy) 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 &gt; 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.
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.