Some grocery stores will hire 15 year olds as baggers, so you could find out about that. If you have an interest that could make you money, such as sewing, making rag rugs, making jewelry, you could do that. It's summer so there's always lawn work to do. If you have elderly people in your neighborhood, ask if they need their garage or attic or basement cleaned out.
The old reliable lemonade stand is always good. If you're good in math, reading, etc., you could be a tutor. Ask mom or dad if they have any friends whose children need help. If you put your mind to it, you could probably think of more ideas. Good luck.