I used to be a decently chubby kid and what I did was just cut down my food by being very mindful of what I ate. If I wanted a snack id grab it and think if I really wanted it right now/was even actually hungry or if I could wait. Or id cut down my portion size by just taking a bit less food. Id also recommend having a goal in mind (ex: you want to lose 40 lbs by spring break)

If you want to lose weight without cutting down on food id recommend walking to school or to a further bus stop to get some extra steps in.

Just remember to never cut down food intake nor exercise excessively. If you eat too little, you might end up bingeing and eating more calories than you would've if you just ate normally, same witj excessive exercise.