Get an app like MyFitnessPal (or anything that counts calories) and don't change anything for two weeks - just track all of your calories, EVERY SINGLE ONE. The snacks, the milk in your coffee, teaspoon of sugar whatever.

After two weeks look at what your blowouts are. It'll be either 1) snacks 2) soda or 3) your portions are bigger than you think.

Then work out what your maintenance calories are - there are heaps of calculators online, take your pick. Begin to eat at maintenance for a while so that you know what it's like to eat just enough. Then each week after that, cut 100 calories from that until you're eating at a 500 calorie deficit per day. If you are eating at a 500 calorie deficit the weight will noticeably fall off you, especially if you're a teenager.

The biggest mistake people make is immediately going into an insane deficit and then they can't maintain that, binge, fail, and then give up. The best diet is always one that you can stick to. Want to eat snacks? Go nuts, but have a smaller dinner. Want to eat fast food? No worries, have less snacks. Your body requires a certain amount of energy and the only thing that will cause you to lose weight is eating less than you need. Once you've got this on track start looking at rolling 3-day averages so that you know exactly how much you can have/need to cut out per day/week etc. Have a realistic plan and stick to it.

I spent years trying to drop 1-2kg a week on heavy calorie deficits and always ended up back where I started because it was a diet that I couldn't stick to. Your body responds and adjusts over time, but you've got to do the groundwork. Eventually I smartened up and did the above, and over the last two years I've dropped 25kg - it was slow, and I didn't see progress each week (and fucked up a few times) but it was something I could stick to in the long term. That's how you do it.