You can earn 600 in a summer if you do work. I agree, you seem way too much of a novice to be riding green horses...pay or no pay.
Anyway, mucking stalls and grooming earns some money if you are thorough, I did it for a good friend growing up and didn't want to get paid because she was a friend. In addition, wash cars, mow lawns, and do weed pulling for friends and neighbors for a few bucks. It all adds up. When I was teen, my biggest cash cow was in petsitting, and I didn't even charge a specific price. People just paid me what they thought was good (and I always thought it was too much). I got all my petsitting jobs through word of mouth because I was dependable and passionate.