What Causes Missed Periods With Gain In Weight?

Hi, im 18 years old and i have a problem. For the pass 2 1/2 years my cycle been messed up and for the last 7 months i have not had a period. Im not pregnant and i know these because im not sexual active, but i have gain alot of weight and everything i do it just won't come off. My friends tell me it could be my thyroids but i don't have insurance or money to see a doctor. Can you tell me anything?