My advice is read, read, read. The best way you can compete against crappy repair services is by building yourself a reputation for good service and know-how. It helps to know how the components actually work, even the physics behind them, since that makes it easier to understand what differences new products implement. Keep yourself well informed, don't believe the first thing you read (read several hardware review sites and blogs).
Build a network slowly: first through friends and relatives and then expand it by advertising and reputation or creating a web site. Friends and relatives are good and bad: they will most likely expect you to do everything for free. This is good at the very beginning, to gain experience, but you should state clearly, at an early stage, that you intend to make a business out of it. Otherwise they will exploit you forever, keep asking you for favours for their friends and demanding special treatment, which is extremely frustrating in the long term. A way to keep them happy is by doing good work and giving them small discounts.
It is good to do some voluntary work at first, since it is great practice, but once you have the chance to take things out of your first circle of friends, relatives and school it's best to move on.
And don't be put off by annoying/rude/misinformed people, because you will encounter TONS of them, and it's not worth sacrificing your effort and enthusiasm. There will always be people that appreciate your great work, and it's those you have to focus on, not the random idiot who yells at you because he/she doesn't know how to exit the screensaver and thinks it's your fault.
I find building computers gratifying, and sort of a craftmanship. I am a physicist, and knowing what's going on in each of those components is great. Each system seems to have a personality of its own, since even with identical components the behaviour is not entirely the same (this can be frustrating sometimes), and everything you build can be a work of art, even if you do it for someone else.
I think you have some great advice from the three posters above, so just my two cents here. I don't work "in the business", but do have some experience (and from a variety of other jobs too), so I hope it helps
By the way, I kind of hate software, so if you do not like it either I'd never offer services past the BIOS, firmware and basic operative system config. After that it can be an absolute NIGHTMARE!