Weight loss?

So.. Ever since I’ve started having sex I’ve started gaining weight and obviously not a lot of people know I also work at a fast food place and ever since people just keep calling me fat, I’m 5 feet and weigh like 140 and I have big boobs and I have a butt and everybody just keeps exaggerating up to the point I really do think I’m fat and don’t feel comfortable with myself at all and I want to exercise and what not but I just have no idea how to start, like what to eat and stuff and what to do, like go jogging or do exercises, I just don’t know 😕