Firstly, all of AUwarrior’s points are spot on and should be read again to sink in a little more.
I will also tell you that I did pretty much what you want to do to get into the business 9 years ago, although probably with more experience as a pretty hardcore hobbyist. I had about 5 grand in tools, and had done bare shell rebuilds on my 92 Jetta I still have and a 86 golf. I had no real credentials whatsoever and it was a matter of talking my way into a Kia dealership. My experience was vast with those cars and the all things I’d done gave me a real experience edge, but learning professional auto repair was an entirely different thing and I knew that going in.
Basic interview etiquette applies more than normal because you’re in the position of trying to convince someone to give you your break. That just means look clean and groomed, speak well and with confidence, look him in the eye at least half the time, and be honest. With no credentials, that’s the only thing the guy will have to go on to decide if you are real or just a mouth. Anybody who has hired mechanics has already heard a lot of talk.
You will want to explain that you are there for a lube/entry level position, but you already know that you will grow beyond that. Bring up the experience you’ve had working on your own stuff, saying how that has already made you comfortable working on cars even though it doesn’t mean you know everything. You want to speak humbly about what level you are at now, but very confidently about your comfort with mechanical things and your desire to learn. They will only think you’re cocky if you act like it.
What any boss wants to hear from a starting guy like you is that you are not set in any of your ways of doing things and you are ready to be molded into a professional. That goes for any job really. That’s the best answer to the famous interview question, “What do you think is your strongest quality?”
“I’m not really set in my ways too much and I can learn to do my job however you, my other superiors, or the facility want it done.”
Of course if you tell somebody any of these things and they aren’t true, it may not ultimately go well. Every shop had different policies and procedures, and some have none at all and it’s chaos. Being able to adapt to those kinds of changes now and in the future makes you happier and more employable. You go in, look around, listen to everything your told, and don’t talk too much. Take the time to pay attention to how things are done wherever you get into, and emulate those practices. It will make you get along with co-workers much easier from the start. A simple example of that is that if all the techs in the shop put the tranny flush machine back in the same place with the knobs in the same positions every time they finish using it, and a new guy just has to keep doing it different, it will always rub everybody the wrong way.
One final broader point:
If you want to grow to the top level of auto repair, You will NEED to get the book smarts that you would hopefully be getting from going to school. You can get them on your own if you are really interested enough that you will read whole books and thirst for more information, answers, and understanding. Your personal repair experience, $500 in affordable books, and the willingness to read them until you understand them can replace that formal education as long as you can teach yourself and want to learn. And for god sakes, learn electrical! Here is your fist book to get. You can’t possibly handicap yourself more in the long run than by convincing yourself you’ll never get it.