Agree with both Chill and Stud on Mia. She has an awesome rep but she isn't known for taking first timers.
Your first time is a little like ski jumping. You can prepare only so much and at some point you just have to do it. The girl is much more important than the studio. And your tastes matter too.
My suggestion. Let your nerves work for you. Screw up your courage and walk in somewhere. If the girl who greets you seems your type and easy to talk to go for it. If not, tell them that you didn't know whether to make an appointment or whether you could just walk in. If the fit doesn't seem there just say thank you and walk out. They see nervous guys walk in and out all the time.
Once in the room, the girl will,at some point, ask a leading question or massage you very intimately. At that point respond positively and say that you've heard that for an extra tip some other services may be available. She will be worried that you are not a cop. Once you get that out of the way this will likely lead to a conversation where you can tell her what you want.
Incall is generally a better experience but its hard to beat the availability of the studios.
Lance