I've always been overweight around the 90th percentile, but right when summer started I gained like 20lbs. I grew an inch, yet it seems whenever I grow, I only gain like a couple pounds. I see why this may have happened since all of my sports ended, but I was wondering what I should do to lose this weight. I'm 5' 5'' (1.65 m) and am 173 lbs (78.6 kg). I want to get down to at least 150 lbs (68.1 kg) by the end of summer. Please help!
Assuming that you are a person in normal good health who wants to lose weight, the best approach for you is to either decrease your intake, exercise to offset some of your intake, or both.
You say you noticed the weight gain when all your sports ended. That does tell you something. Eating to maintain energy and strength during sports is physiological. Eating the same amount once you stop engaging in that sport is going to add weight.
There are all kinds of diets out there which, usually for monetary reasons, claim superiority over one another. You don't need to read about or follow any one specific diet to lose weight. If you decrease caloric intake over a sufficient period of time, you will lose weight.
Reduced-calorie diets result in clinically meaningful weight loss regardless of which macronutrients they emphasize.
One approach you can take is to eat the same stuff you eat right now, but eat less of it. If you eat a Wendy's Baconator for lunch on Saturdays, leave some of it uneaten. Leave a third of your shake, a third of your fries, etc. At home, eat a smaller portion of everything equally. If you consume sweets, like soda or other nutritionally empty calories, cut them out or cut them back.
Exercise. Take walks, take hikes, ride a bike. A male your age probably plays video games. Cut back on these, and do something which exercises your body instead (it doesn't matter what; listening to your summer reading list on audiotape while walking/bicycling/etc. will do the job.) Learn to swim if you don't already know, and keep in shape this summer by swimming regularly.
If you have medical problems, have other symptoms of concern other than just weight gain (e.g. fatigue, constipation, a change in your voice/hoarseness, weakness, etc.), or any other symptoms of concern, see a doctor. If, after an honest effort was made to lose weight, you are unable to, you can see a doctor for more specialized help and to determine if you need testing.