You have different possiblities:
If your program is already running
Press CTRL+Z to suspend your program. It will not be running now. To resume the program in the background, type bg and press ENTER.
If you want to start a new program in the background directly
Simply add & to your program before you start it:
$ geany "do_while.cpp" &
You might find that the program outputs status or error messages on your shell. These messages will not directly affect the commands you type, but can be very disturbing. You can suppress/discard this output by redirecting it to /dev/null:
$ geany "do_while.cpp" 1>/dev/null 2>/dev/null &
For more information on job management in the Shell: