Babysit? Try to get a job at a barn...that way you might gain a little more experience with caring for horses
Hmmm...what else? You might be able to find a job even though you're only 15. I'm not sure where you are from, but when I was younger I was able to get a job at 14 as long as I had working papers (got them from my school) and only worked a certain amount of hours per week...might have only been 10-15 hours, I can't remember. I worked at a local ice cream shop.
Is there anything that you could do around your house that your parents might give you an 'allowance' for? I know...everyone's parents are different, and some are not into this!
Ask neighbors if they need any help...cleaning up the yard, mowing the grass, pet walking