Best way to loose wight I'm thirteen ?!

You probably shouldn't try to lose 20 pounds unless you're SEVERELY overweight, which I'm guessing you're not, judging by how into sports you are. Maybe you should just try to tone up a little and improve your fitness?
I'm 13 too and the best way to get in shape for gym class and lose a bit of weight overall is cardio... I do 45min-1hr everyday, but if you're not used to it, you can start at about 20 minutes. Stay involved in sports , but make sure you have one day per week to just relax and maybe do some relaxing yoga on that day to stay flexible and give your body a break.
Anyways, weight loss is 80% diet, 20% exercise, and barely 1% genetics. So diet is really important, but don't go for any difficult diets, just try this. Eat healthy throughout the week, according to the food guide. And eat less processed food and more natural raw foods like fruit. Eat fruit instead of junk food when you're hungry, and only eat junk food on the weekends. It sounds really hard, but I actually like doing it that way cause then it's so rewarding to know that you earned it by eating totally healthy through the week, and you don't feel as guilty... Also make sure to eat a big breakfast to keep up your energy and if you can, eat more small meals more often during the day because it keeps your metabolism up. Drink water more often instead of soda, even flavored water is okay. Also, juice is a good way to get up your servings of fruit, and of course, you need a glass or two of milk per day. and More water helps you feel more full, so drink 1-2 glasses before each meal, that way, you won't be eating quite as much.
hope I could help (:

I'm also 13 and looking into weight loss (:

You have to challenge yourself. If you're doing the same thing every day you aren't working much off. Try running faster or longer.

Also consider that you might be perfectly healthy, and there might just not be any weight to lose. You sound active, so I have a feeling that's what it is.

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Drinks lots of water!
I read about this diet where you actually drink one glass of water before you eat, one while your eating, and one after. Might sound crazy but I tried it out and it actually does help you shed pounds fast!