wow, for you i say start from ur house so you'll lose less money. doesn't sound like u know what you're doing. pay someone or learn.
maybe quicken or some other accounting software, i think thats the first question.
customer relations solution: phone, email, be available, or hire customer service (or repair) people, web site.
repair info can be found online or thru various books. we do have pc sub-forum that would be more appropriate for you, read that for now.
edit: now that i see your earlier post (double post) i see u want a car repair shop. start as shadetree mechanic, craigslist, build clientele.