I lucked out on craigslist. If I'd known what I was doing, I'd have gone on TER, made a fake account, and then asked about agencies. Most agency websites have an email you can use to apply (they all want full-body pictures and a good face shot), and then you just interview like for a regular job. Mine was a phone interview because my agency is busy and I live a little ways out of town.
Reputation is EVERYTHING in this business. Mostly, it's based around the hobbyist community on TER and several other secret forums. So, if someone wanted to see a new escort, he'd look her up on TER and read her reviews, which have a numerical component and a description of the session. Because of that, getting good reviews is the best publicity you can have, and someone who is known to be a TER reviewer usually gets the VIP treatment (nothing special, I'm just extra-nice and am careful not to make him feel rushed).