Try and improve your diet would be a great start. Try and make it balanced, avoid fatty foods and sugar, get some carbs, protein etc.
Join your local gym, a personal trainer will usualy give you some free advice.
Personaly I think you might be a bit too young to get "Buff"... most gyms require you to be 16 before you can use resistance machines.
Other option is to work on cardio, do running/cycling/swimming until your 16, then start working on your muscles.
Personaly I get up every mornining and do 100 crunchs, Plank, etc to work on my abs, cycle to the gym, work on resistance for 40mins and then do an hour cardio, cycle back.
Try and intergrate excercize into your day, as well with the better diet and most importantly REST, which is commonly forgotten.
Lastly, dont stress about wieght at your age... just try and keep active and healthy, dont let your focus on school drift away
