First of all, you need to do exercises every day. Runinng is also a good way to keep fit. It is also interesting to go swimming in summer and skiing in winter.
Pay attention to your diet, you need to eat healthy and nourishing meal. Don't eat fatty and junk food. You must eat in the particular time and mustn't overeat. Drink a lor of water.
Correct daily routine is also necessary - get up and go to bed early, walk a lot.
Invite your friends to keep fit with you - it will be more interesting and motivating together.
Don't let others bring you down and believe in yourself.