I want to lose weight, whats the best way of doing it?

hi im 16 and want to lose weight im currentlya size 10, and weigh about 9 stone. i dont think im really fat, but just want the podgey belly and thighs to go lol. I put alot of weight on using the pill (birth control) and have tried going to the gym for 3 months every day. and didnt drop a pound, im now on a new birth control injection, after i got pregnant on the pill and suffered a misscarriage, im starting to get frustrated. Anyone been through this, or got any good tips?

thanks xx

Ayla-Marie