I am 13 and I want a flat tummy, my mom said to do situps. How many do I need to do?

I am really self-conscious about this and I would really appreciate it if someone could help me. I need to know how many situps I need to do and how often I need to do them. Also, if this is not the right kind of exercise for a flat tummy, please tell me what else should I do? I don’t have any money so I can’t buy exercise equipment. Thanks for your help!