Not familiar with sonar. Just looked it up and it's a Cakewalk product. Ok, there should be a VU meter, it goes red when you sing in to the microphone. There should be a dial or slide that is labeled "Level", "input Level"or something like that on the channel you are recording to; slowly turn that down until the VU meter no longer goes red. Then turn it down another 2 or 4db depending on how wild it's going to get. Try that and see what you get. You could also increase the distance to the microphone when you sing if you can't figure out the controls, but you'd sacrifice on clarity of that great voice.

P.S. I also have no formal training, I just tinker with machines until they either work or blow up... I blew up a few so don't let me near yours, LOL!