experience?

I started out in my back yard, so to speak. Got more and more experience through trial and error. Bothered people at shops for info and help.

Eventually I took my car to a shop because I couldn't finish something, and I wound up a helper there for some time.

Your car and your friends cars are prime targets for starting. Just know that the game drastically changes from the back yard to a real shop. There is a lot more responsibility and pressure.

The best way to learn is by doing- either working on your own cars or friends/ family members'. 

You can go here to take a practice test: http://members.ce.org/certifications/mecp/exam/practice/basicinstall.asp?ID=2322307.  It won't give you any personal experience, but you can pick up a study guide ( http://www.amazon.com/exec/obidos/ASIN/1588870030/the12voltcom-20), and take the Basic Installer test at your local library.  Much of the basic test is simple Ohm's Law questions- you will have no problem with it. 

That will help you get a start at a store like Best Buy or Circuit City (doesn't sound very glamourous, but it's a start and will give you some experience).  Sometimes you'll be surprised at the skill level of these installers because they've worked at custom shops before that were commission based, but didn't get enough labor.