You have three options for car detailing training.
Your first option is to train on the job at an existing detailing business. You must learn not only the responsibilities of the workers, but master the tasks of the owner himself. It's obviously very difficult to find an owner wiling to "open his books" to a potential competitor. Thus, you must approach shops far from your area and possibly work without payment.
Your second choice would be to train at a school. Schools produce working detail business operators. And the good ones training great sales and marketing professionals--not just car detailers. If you do choose to attend a school, pay close attention to the marketing advice of your teachers (ex detail shop owners). They have fallen into all the traps of a new business, and their advice can save you thousands of dollars.
Expect to pay $1100 - $3000 for schooling depending on your travel expenses.
Finally, it is possible to train on the Internet to open a detail business. Some detail schools are offering online training. There are websites that fuse book training with video courses. Some websites go into great detail about website marketing--the most important new frontier for new car detailing customers.
Whatever resource you choose, the most important lessons as a detail business owner will be learned on the job, usually making mistakes. But getting the right training from the beginning will minimize the severity and frequency of those mistakes. So choose the option that fits your time and budget needs, and dive in!