My best advice to you is to read read read everything you can on here and every detail or small business related website.
Most small business owners are good at what they do (i.e. cleaning cars) however they are not good at running a business. As you can imagine there is a huge difference. Cleaning the cars, correcting to paint, dressing the tires, etc., is the easy part. Finding clients, and enough to keep you busy, is the hard part.
Dont get discouraged but also realize it isnt an easy job either.
Now to answer your last question you posted:
I always have smaller jobs priced at a near set price or at least a small range. Figure out what you want to make per hour and price our your smaller jobs based off of that.
For paint correction jobs, I would always give clients an hourly rate and estimated your time you think you may spend.
I would be glad to help you out as detailing is paying for my education right now. PM with any questions.