This is (not-so)short version of the weight management basics I give people. I know it's a lot to read even in its condensed form, but I know for sure this information is what you need to get started on your goals.
Where you are now it's very possible for you to lose weight without much exercise. It'll only take you so far, but it will be the basic principles you learn now that will help you in the future.
Weight loss is really simple. So simple that people over-complicate it.
You have to burn more calories than you take in.
So the very first thing I have people do when they ask me how to lose weight is to calculate their caloric needs. You're still probably growing if you're only 17, so the calculators you can find online are going to be more off than normal, but the important thing to remember is that any calculation you get for caloric needs is just an estimate. You're going to have to play around with the numbers to see what works with you.
Very simply and quickly, your body has a set amount of calories it burns daily just to function. When you multiply this with your activity level you get your total caloric needs.
In all actuality, you can eat whatever you want and lose weight so long you eat under your caloric needs. Now, don't take that as me saying to eat nothing but McDonalds all the time. Can you lose weight doing that? Yes. Is it a smart thing nutrition wise to do? No. But I often include ice cream and chocolate into my diets no problem.
To be honest, you need to. If you deprive yourself of such things, like quitting cold turkey, you're probably going to binge and hate yourself for it. You'll end up taking one step forward and two steps back.
But let's put all of this in a real world setting.
Let's say for your age, weight, and height that your BMR is 1200. You're sedentary, which means you sit around and do nothing, so that's going to mean that you'll need roughly 1400 calories a day to stay the same weight. But let's say all of a sudden you start exercising, let's say 3 days a week, that pushes you up to 1560 calories to stay the same weight. On it's own it may not seem like a big deal, but let's combine diet and exercise together. So if you were normally eating 1400 calories a day and started walking 3 days a week, you'd pretty much lose 700 calories a week. Now, combine this with proper dieting and say you only eat 1200 calories, you're not creating a much larger deficit.
That's why diet and exercise combined is the best goal.
But let's say realistically you probably need to eat somewhere between 2200-2500 calories a day to maintain your weight currently. Without any exercise, you could cut back 300 calories a day or so and see slow and steady progress.
But this is why calculating your needs is the very important first step. It's the most important step in my opinion. Sure, a lot of people do fine with guess work, but it's much easier to create realistic goals this way.
This is just a start, and if you'd like more detailed explanations then I'd be more than happy to share. We can debate on what you should eat, how often you should eat, and so on.
But the only important thing is that you remain in a calorie deficit. That's the only way to lose weight. Eating one meal or ten meals a day doesn't matter if you're eating above your needs.