Since you are in Jr high its very unlikely you have a car of your own that you have practiced on . It is likely that you have practiced on your parents cars though so i wont take that away from you. The idea of a person in Jr high using a polisher, even one as forgiving as a GG, on a car for pay is some what unsettling. Not saying that you are a bone head and will burn though the paint or sling wax every where as i dont know your skill set. What I am getting at is a lot of people are gonna see you as a kid with a grinder to their paint in my opinion. Plus the cost of a polisher and pads youre upwards of $300 so you gotta figure how long it will take before you see a return on your investment. So here is my advice since i have brothers around your age.
No need for a business license as you arent really going to be a business. Id throw up an add on craiglist saying $50 wash and wax most vehicles. Then id print out some flyers and put them up on the cork board at community centers, grocery stores, and even pass some out to your teachers. But then again how do you plan on getting around and transporting all your equipment? So heres another idea. Print out a bunch of flyers and knock on your neighbors doors and tell them who you are and what you are doing. I think thats your best approach.
Now in your position as far as products go I would go fairly cheap on this. Meguiars makes some very easy to use stuff. When i say easy i mean if it gets on trim it wont leave a chalky residue. Id pick up a fairly nice wash mit, 5 gallon bucket, stoners glass cleaner, a decent shampoo, micro fiber towels, wax applicators, LA totally awsome, megs nxt gen wax, and a run of the mill tire dressing. All that together will be around $70 to give you an idea.
Its totally fine to use a customers water and power. Just make sure to give them advanced notice. All you have to say is "is there water and power on site" or something similar. Some of us who have been doing it for a while and who are a legitimate business still use customers water and power and have never had an issue.
I hope this helps