Make sure you're in your 20's, buy quality tools then stick with it (this includes the polishes and compounds and don't keep getting the latest and greatest), read/watch all you can absorb with regards to detailing, work in a clean environment with good lighting, work with real expectations, drink lots of water.Any advice for someone who wants to start doing professional level detailing to their own car?
After all said and done, it is a labor of love when it comes to your own car, so spend as much time on it as you see fit. After some time and enough cars, you can venture out to cars of friends/family, and they might even pay you for it, LoL.
A good random orbital dual action polisher is your friend. Don't move on to a variable grinder/polisher until you're comfortable using the DA polisher. For newish cars, a random orbital is all you will need + various grit polishes. I'm a Meguiar's guy but go with what you prefer/can afford.