In California, any job that bills less than $500 does not require a license. However, I'm with skip555 in that you should probably have some kind of protections in place.
At least in California, a state approved license training center will run you between $1,200 and $1,800, depending on what bells and whistles they offer. Most guarantee that you will pass the test, and trust me, you WILL learn a thing or two (especially since about half the test covers the legal stuff about business). The more expensive ones will handle your test fees, state paperwork and all that junk. The one I went to included a state-required bond of $10,000 for basic covereage. I chose this path so that I could get going immediately without a second thought. The whole thing will take you eight weeks if you work it straight through...just a couple of hours on a Saturday for formal training, then regular visits to take practice tests.
When all is said and done, you'll want to frost the cake by incorporating your business. If you don't know any friendly attorneys, there are online places like legalzoom.com, where you can expect to become incorporated for around $500. God forbid you ever get sued; but if it happens, they'll leave your house and other personal belongings alone.
Trust me. This is all good advice.