Any tips on how to lose weight (fat)?

I'm definitely not fat or anything, I'm average I think but I noticed I'm starting to gain weight and last week I went on a trip and gained a couple pounds, and my thighs are thicker and I have more belly fat than usual. Any tips on how to lose weight? I am under 18 but I have stopped growing, and all i do is gain these days.
Any tips on how to lose weight (fat)?
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