Hi @Iaoaoe and welcome to the DCUK forums.
You are very young to be worried about weight gain/loss so the first question I have to ask is
Are you technically overweight? (It is normal to gain weight as you grow into your adult body and yes, that includes needing larger clothes.) Though I didn't realise it at the time (and neither did my GP or my endocrinologist) I was actually underweight between the ages of 15 and 18 and my health actually improved when I went up a size at about your age.)
Yes, there are things you can do to lose weight but as you are still doing important things like increasing bone density I would be very cautious about nutritional restrictions until you are older.