Going into same line also, however I started while still working a full time job to learn the tricks. Others answers are important - also: I travel with a group of Utilities for t-shooting that I know well (my prime Norton System Works - choose whatever you like, but don't learn at the customers expense, know it and it's limitations before you use it on a customers machine), also OS's (NT, win95 a,b,c, Win98, WinME, and upgrade versions). I also keep a HD with the OS's loaded - system commander to select, and current Norton and MCAfee Anti-viruses: this allows AV checks on customers systems, data recovery, and eliminates their software or bad driver loads from causing system lock-ups and boot problems. Keep old parts i.e. 1 meg video cards - great for showing cust. what fixed prob, can also hold them over till bad item can be replaced, can't play games but at least they can read e-mail. Also old systems can be used to plug parts in to verfiy bad parts. And a good work bench in case it can't be fixed right away. My hourly rate is also the 1st 30 min on a house call - to cover gas and vehicle expense.
Make contracts - what you are and are not responsible for, not all customers will treat you as ethically as you treat them, but they are right.
Get A++ and MCSP's or MCSE to pick up small businesses.
Be straight up, if it's software you don't know - tell them, then explain the time involved in learning it to help them.
Good Luck, feel free to email me and we can swap warstories.