As preparation you would like your hair to be trimmed about 1/2 long so that the wax can grab hold of something. Any longer will hurt more and any less will need to be waxed a few times over to get it all off. I recommend not looking at your junk until she is completely done; at least for the first time when you are a bit nervous. The first time hurts the most, at least it did for me. By tour tenth time you'll be looking down, noticing missed patches while giving feedback, and talking about the weather as your hair gets pulled out; you'll be a pro then.
Other than that (1) bring lose underwear to wear afterwards, (2) don't take a hot showers afterwards to remove the excess wax, and (3) use some Bikini Zone after so that you don't get ingrown hairs. These are followup stuff.