Finish high school first. Never burn bridges and at this time you wouldn't want to do that with your parents. But in the mean time find a niche in a space and devote 100% of your extra energy to it. When I was 15 I had just finished rewiring my entire high school's ethernet system. I brought on 5 other kids my age during the summer and we had guidance from some older staff who showed us the ropes and let us go and do whatever we had to, to get the project done.

Throughout the whole process I learned to ask question and not be afraid to try and fail. My point to telling you this is that with $500 you've got very little to lose. You can always deliver pizzas and make that back in a few weeks. So don't be afraid to try something really crazy to get you somewhere. Also remember to ask for help, advice and be HUMBLE.

Read about starting businesses online, I would say read some books, but with such a limited budget you're better off saving your money for your business. Don't forget to take notes! It helps retain the information you read.