Best to go through some of the tutorials at the start of the Reference Guide to get yourself up and running. It won't take long and you'll get a much better idea of how to move around the program. This could be any number of things that would likely be addressed in that section.
However if the mic and it's driver is being recognized by Sonar then you have to set the track input to recieve input from the mic, turn on the track's Echo button (which should give you audible input and a meter reading so you can set levels), arm the track for recording (press the Record Arm button on the track control area) then start recording by pressing the Record button in the Transport Panel at the top of the screen.
As you can see, even though that is relatively simple to do once you know the basics, you gotta know the basics which the tuts will show you. If it still doesn't work then we can help you troublshoot from there.
Cheers and welcome.