Going on the pill because your friends are on it is not a good reason. I know it seems like it when you're 15, but believe me, you'll look back at this time and wonder why you didn't think for yourself. I don't want to preach, but don't be pressurised into anything just because your friends are doing it. (ok, preach over!)
If you want to go on it to help painful periods then it can help some people. It didn't help me, and it made my stomach hurt very badly every day of the month rather than just before my period, so I stopped after 1 month. It's best to discuss with your doctor.
If you want it for contraceptive reasons then that's sensible as it will protect you from unwanted pregnancy, but remember it won't protect you against any sexually transmitted diseases, which can be very serious and painful and may even kill you. Use a condom whether or not you're on the pill.