When you sing from your stomach or rather your diaphragm, the sound is usually stronger and your chest doesn't hurt as much. Usually you do that when singing a long phrase or loudly so you don't run...
yes you can but be creative with your song. but first of all you and the girl that you like have to be alone because singing it with alot of people around will pressure her and make her feel more shy...
usually when you sing through your nose it sounds very nasal. or some tests you can try is singing a song and if you are not breathing when you are singing notes you are doing it through your nose...