I'll tell you what I am looking for in an installer first then I'll give some advice. You can take it with a grain of salt but youo asked for it. A good candidate to be an installer would have these attributes.

Appearance:
Short hair (not dyed in anyway, not even frosted on the tips)

No earrings or tattoos (If you have an earring take it out if you have a tattoo cover'em)

Cleanly dressed

(reasoning: If you look like a freak people are usually not comfortable with you in their home. Alot of times the wife is the only one home and you scare her!)

Communication Skills:

Have the ability to speak the english language without saying things like (whass'up, ummmmm, like you know, I'll holler at ya)

Remember when you are in this business you will work for people that have more money than Jesus Christ. They expect certain things in their home. They expect an installer to be polite and courteous. (It's okay to hate them when you get home)

Knowledge:

You need to know everything!!!!!! You cannot not know the answer to a question. (this comes with time, there is no way around it)

Advice:

Depending on your age I would consider applying at a major retailer ( Circuit City, Best Buy, etc.)
This will give you the knowledge of the majority of products. You can learn what the difference between 5.1 and 6.1 is, or for that matter what 5.1 is in the first place. You can learn the proper way to hook up components. You'll have a good grasp of the basics.

Then, you've got something to deal with to an employer. "know sir I can't fish wire to save my life but I can hook up components and program universal remotes, cable management, etc.

An employer would rather hear you can do 1/2 than nothing