I doubt you are old enough without parental consent, considering it's a stranger dealing with your "private parts".
A hollywood removes ALL of the hair down there, and a Brazilian removes all but a small patch at the top. If you are a man, and have never had anything like that done before, you're in for a little bit of shock pain, and it really depends on your tolerance level.
you should let your hair grow out a tiny bit before you go though, that way there is something for the wax to bind to. If it's too long, they can just trim it, don't worry about that.
as far as the erection goes: think about the news, lol...because some people get in a happy mood out of the simple fact that they are exposed and with another person, and if the person is good looking, well again, think about the news or something, lol.
p.s. I would personally wait at your age...but it's your pubes