my high school offers a half-semester weight training class. after that i just started doing sets of 20 pushups, 40-60 crunches, 10-15 pullups, weight lifting, all that stuff.
no weights? no problem. fill your backpack with rocks or something else heavy. if you want to get rid of a gut i'd start doing a lot of running.
all of this is in vain if you don't eat right. start really looking at what you eat, and don't overeat. a good way not to overeat is to take a bite, chew it thoroughly, swallow, and wait like 10 or 20 seconds before taking another bite. eat moar meat and eggs, buy whey protein and make shakes.
once you think you're ready try going to your school's weight room. come with some buddies if you want so you don't look/feel awkward. a good basic workout to start with:
- bench - 5 sets, 7 reps
- incline - 5 sets, 7 reps
- squat - 5 sets, 10 reps
- BB curl - 5 sets, 10 reps
once you feel comfortable adding weight move on to different exercises such as front squats, rows, push-press, skull crushers, etc.