You're in the right place. I admire your willingness to learn. My advice is the same as Celeste's..here's why. People will expect to pay you next to nothing. At 15, how are you planning getting the washer around? I guess you could get a portable and walk around but there is only so far that you can go. By the time you get a licnse and insurance, a machine, a few neccesary attachments and hose, I doubt you'll make your money back. Get a job with an established company. When things go wrong, and they will, you can rely on two things.. the insurance of the company you are working for and their insurance. You will get started right by learning from someone that has been doing it awhile. If you go on your own now, it's trial and error. And yes, your parents would be responsible if you mess something up. It sometimes takes a trained eye to walk away from a job where you know you will do more harm than good. You need to get that experience.
You will be so far ahead of the game. You will have picked up business tips, proper technique and chemical usage, you'll see a variety of jobs and by the time you are ready to start this thing at 18, you'll be the man. Seriously, you have to trust those of us that do this professionally, you will make more money working for someone.