The point of this websites (and others) is to collect information and disseminate it.  Search the review database for providers that have high marks and fits your type.  Since no one really knows your type except you, you'll have to search and check out websites to see if they have some good information that will help you make the choice.

Since you will have no referrals for verification, you might start with Room Service 2000 and get in their verification database.  After getting that taken care of, make sure you have the following pieces of information handy:

1) Date, time, length of the appointment.
2) Location (incall?  outcall to where?).
3) any information for last minute changes, like email/mobile phone.

If they have prices posted to their websites, understand that its generally fixed and they will not lower them (unless you saw an ad in the message board here or elsewhere).

The point of having all this information ahead of time is so there's less back and forth when communicating via email or voicemail.  There's no point in wasting their time by trying to figure out when and where they need to be to see you (and they are not in the free phone sex business).

In any case, if they have a website, they usually have a preferred method of communication.  Sometimes, for first time clients, it's a webform for you to fill out with the information they need.  Others, it's either an email address and/or phone number to call (sometimes with response info, like it'll take a few days to check email, or leave a voicemail and get a return call a day later).

Finally, make the appointment a few days in advance at the very least.  Have a couple backups contacted so if your first choice is unavailable (or worse, flakes out), you have other options.  The reviews and message boards are a great source of info on whether a particular provider is reliable or not.

Power of the Internet, wow...
 Bob