run everyday
start doing compound lifts in the gym, google them, or the guys here will reccomend a good compound routine
do some research into muscle building diets, the guys on here im sure will do it for you wen u post ur current diet which probably is horrible
and LASTLY ur gonna have to wait until u hit puberty before u start seeing alot of muscle and lower bodyfat, at 15 ur just starting the process of wats going to be a fun mix of random erections, acne, hair growing everywhere, and muscles coming from everywhere and willing to sacrifice a night with ur mates just to try get a handjob HEHE