What everyone else said.
I retired 7 years ago and I'm starting my 6th. year in the business. I'm running two crews and spend most of my time bidding and in operations.
Here's my addition to the excellent aforementioned advice;
1) Avoid all consumer debt related to the business if at all possible.
2) Your first customer will be the hardest.
3) Be careful while bidding one-off jobs. Clean ups always take longer then you think.
4) Cultivate relationships with other trades. Especially arborists.
5) Keep your prices professional. You're doing ok if you're catching 50% of the bids.
6) Learn the signs of a possible PITA and avoid them at all costs. They're time consumers.
7) Do not be afraid to fire a client...preferably after they pay you. Be civil, diplomatic, and firm but if your gut is telling you that you're dealing with a potential problem, then walk away.
8) This is better then joining a gym.....have fun!!!!