How to get a girl in high school?

im a 15 year old guy going to be a sophomore this year after summer. ive never had a girlfriend before and i always thought it was cause i was chubby so ive been lifting weights and dieting . im 5'9-5'10 at 190lbs but im still losing weight. i didn't get the chance freshman year for a gf cause i was expelled 3 months in for smoking weed. im always awkward when i talk with girls and i freeze up and don't know what to talk about, but it works out cause im more of a listener than a talker.i don't get why i don't have a gf, girls ive talked to say im a nice funny guy. and when i finally find a girl i like im always too shy to ask her out. and whats worse is i see guys at school with nice girls and they treat them like crap/or cheat on them. im really just lost in the whole girlfriend topic