Ask your parents if you can start buying more fruits and veggies! Just ask them when you're at the grocery store the next time... When you have a snack, choose a fruit or veggie over something like cookies, crackers, granola bars, pretzels, chips, bread, etc. Also, if you want to try to actually go on a healthy lifestyle journey talk to your parents about possibly going to a nutritionist or joining a gym or something like that :)
Diet is always the best way to go, but do not limit yourself to certain foods unless your allergic, Moderation is key! Exercise is always the best way, increases endorphin release aids in feeling good about yourself too!
Don't eat junk food and just get more activity. if you really want to lose weight, write down what you eat it helps more than you think. drink water and if you want to drink soda drink diet cause you don't want to drink away your calories for the day. bring your lunch to school too so you know how many calories are in your lunch instead of the greasy school food. and just walk if you can't workout one day, its better than nothing.
watch the biggest loser for inspiration too ! haha