you’re 15 yes? would it not be be better for you to do say kennel work? Look at petsmiles or the jobcentre website, round your area are a few kennels, or look at vets as well.
You are going to have to look at start up costs, business cards, insurance, etc
Obviously you havent a car, so how would you transport the dogs if anything should happen and you need to get them to their vet?
Most dog owners who want their dog walked want it done during the week during the day, how are you going to accomodate that?
Just some points to think over, at least if you work for an employer you wont have the extra expenses