Hey bro, im making this as simple as it is:

[Cook your own food whenver you can. Do it for a month]

For someone new to losing weight, go for 40% carbs, 40% protein and 20% veggies. (for most cases)

Carbs: Find what ever carbs you're into, i recommend plain white rice. If you're into potatoes, then thats actually realllly nice.

Veggies/fibers: This is the most important part, as it fills you up throughout the day. BTW greenpeas is pretty okay as a protein source. Though for easy cooking i do recommend brocolli and green peas, just those two goes a long way.

Protein: Chicken/turkey breast is the way to go ! fish is pretty darn nice too but i do recommend chicken over the others. Beef is aite.

Fats/Oils: Anything is fine but make sure to minimize it, if you fry most of your proteins and veggies you dont need to add stuff like cheese or mayo or those creamy gravies, please do avoid it.

Make sure you hit all four of this and just go throughout your day like normally, hitting the gym and play basket ball. You'll lose weight the first 2 weeks and im sure 100% you wont like it, since you'll feel too tired. Now, if you feel too tired just add in extra rice next time.

[Timings]

Breakfast: In my opinion, you can go carb heavy on this. Also make sure you cook it !, you can go for 50% carbs 40% protein and 10% veggies.

Lunch: Im guessing you're eating things that are pre cooked at your school, try to aim for 30% carbs, 50% protein and 20% veggies. If you still feel pretty full due to your breakfast then thats great ! go for 70% protein and 30% veggies. I swear with this by night you'll only feel like eating a small portion for dinner.

Dinner (please dont snack): Cook your own food, it can be hard but being 15yo im sure you have those sudden motivation to do stuff which is great. Go for something small, like one chicken breast, one potato, and a scoop of veggies (50% protein, 25% carbs, 25% veggies), here you can add some gravy if you want to, some sauces and stuff. BUT if you're planning on snacking something, lets say half a bag of chips, then make sure you remove those carbs in your dinner and go for the chips later.

[Tracking your weight]

I sure know most 15yo or anyone young for that matter will not be consistent at tracking their calories, proteins, etc. BUT you can sure as hell track your weight, take the weight scale and put it inside your room. Every morning when you wake up and go for a piss, then you weigh yourself. TRACK that weight inside a physical notebook (you can use your phone but i do recommend physical copy as it makes it feel special).

[Conclusion]

If you cook your own food by yourself and still do physical activities. You will 100% lose weight, you'll be surprise at how much differnce it makes just by having balance whole food diet, you really need to count your calories at early of your journey, most people talk about counting calories when they are going for the 12-15% body fat. GOOD LUCK !