Yep, Fred's got it right. I might add that in the area of "customer service", don't just ask the new homeowner what they want where; walk them through the house, asking them to visualize their completed house, and where everything fits and works for them. Then suggest items they might not know about, or have thought about. Additional indirect lighting, 3-ways, small can lighting, LV lighting, all the things that most people having houses built never think about or know about being available. You impress the homeowner and the GC, and make a decent amount on top of a standard install package with the small extras. I still get a lot of referrals from people I've done this with through the years.
Take a few classes offered by mfg. reps, especially lighting, and some late reading of catalogs and spec sheets, along with a little practical experience knowing what it will look like, and you provide a good service that makes you money to boot.
Good luck, and ask anything!
