If you are a 149 pound 13-year-old and have a little belly fat how many push ups and sit ups should you do per day?

It's not about push-ups and sit-ups. It is a about getting regular cardio exercise. Just do things you enjoy that are also good exercise such as jogging, swimming, playing sports like Basketball and football with friends, hiking, taking a brisk walk, etc. The next step is what you eat. Just avoid dangerous weight loss plans and focus on getting healthy, not the number on the scale. Make it a fun project, and treat it like a marathon... not a sprint.