Ive been lifting for years and have done well. I used to have string bean arms and a chicken chest. You need to eat well and eat enough. Keep the junk food to a minimum. Eat lots of good protien several times throughout the day and eat brown rice, veggies, oatmeal things like that. Im assuming that you have access to weights in school or at home so heres a routine you can do. Remember less is more!!! always let your muscles rest. Mon, Wed, Fri do this routine for starters
Squats or leg press
pushups or bench press
standing curls with dumbells or a straight bar
do this for a month then change things around a bit....add more wieght as you get stronger add more wieght
do 4 sets of each exercise