before you get a flat belly, you have atleast reduce you weight to come to normal limits for your age .
You can start with exercise by joining a gym/ or starting with a sports activity like badminton/ swimming which is a gud way of losing weight and attaining fitness. Diet has to be optimised, for which a dietician can help you.
Also, if there is a medical problem, like irregular menses, that has to be addressed too. ( to rule out PCOD