Q: What is the best exercise for a 5' 90lbs, 13 yr old to do to lose belly fat?

A: You height and weight are both perfectly healthy for your age, so you should not have to do anything special. You're not supposed to have ripped abs at your age and some fat is perfectly normal and healthy. Do NOT try to lose weight -- that's about the worst thing you can do. It can stunt your growth and your body may never fully develop as it should. Instead try to be physically acive in any way possible (sports, running, mowing lawns, shoveling snow, etc.) and make sure you have a healthy diet that's centered around tons of vegetables (especially green stuff like lettuce, spinach, broccoli, green beans, etc.) and whole fruits and not around junk food. You can also start with light-to-medium weight lifting under adult supervision. Just don't push yourself too much in the gym -- an injury now can have that same growth-stunting result. But the main thing is just to be active, eat healthy, and grow! If you lay the foundation to healthy lifestyle now, you'll enjoy the rewards (including having a great physique) for many years to come.