Hello!
Wow! Good for you! Welcome to the world of doing what you love--plus doing what you hate--ALL the time. I have a small tutoring business that will be eight years old next month. My philosophy on small businesses is that they are like toddlers: turn your back on them for a second and they will have gotten themselves into trouble.
The worst thing about being in business is that when things go wrong, it's all your fault.
The best thing is when things go right, it's all to your credit.
I can't imagine ever working for someone else again. This little business is my baby and I'm soooo proud and satisfied (and frustrated) by the challenges it brings.
My advice to you (and maybe you've already started these things):
1) Get accounting software right away. If you're not the accounting type (I'm not), hire someone to set up the accounts if you can afford it (I didn't and my account names might confuse a professional accountant). I love Quickbooks, but it's the only accounting software I've ever tried. I think there's a current post here on Quickbooks vs Peachtree that you might check out. Quickbooks is very easy to use. I really recommend using the online version (www.quickbooksoe.com). You can access it anywhere. Your computer IS going to crash and die some day. Not IF, but WHEN. Having important accounting and invoice information stored somewhere else, with a company whose job it is to protect that info, brings me a lot of relief.
2) I'm into the whole guerrilla marketing concept (www.gmarketing.com). Be creative. I got rolling with some of my biggest clients by volunteering a few times at local schools. I don't know how that translates for a mechanic, though.
3) Looking back, I know that cold-calling works. "Hi, this is me and I do this. Can I drop off some information? Can I donate to your school's fundraiser?" You only need a few to get the ball rolling.
4) Customer service is king. Know how to say "I'm sorry" when something goes wrong, even if it's not really your fault. However, that might be a different issue with a car. "I'm sorry I forgot to put oil in your engine" might not work so well. When needed, I refund money or comp the client a session if I believe that the company did something wrong.
5) Know when you CAN'T help someone. Maybe the client's request is out of your range. Maybe the client is simply too much of a nuisance despite your best efforts.
6) Try to pay attention to what you're NOT good at and hire someone to do that as soon as you can. I'm creative and can teach almost anything with a 3 x 5 card, but I'm not organized. Having a part-time office assistant has really helped. I can FIND things!
I remember my first two years were spent working working working and thinking almost solely of this company. I worked every hour I could get and then brought on another tutor, and then another, as demand increased and word got out. That said, this isn't terribly conducive to maintaining a relationship. (Are there polls out there to link entrepreneurship with divorce?)
I don't know how much of my experience translates to your business. I know that I am very, very loyal to my mechanics. I stay until their services no longer apply to my car. My mechanic helps me to nurse my aging 1994 Mercedes wagon. He does nice little things like screw the door panel back in when it starts to sag. He always sounds like he recognizes me.
Good luck and keep in touch! As a sole proprietor I often feel like I'm on an island. Wave to me from your island! These business forums are great!
:^)