Big decision, props to you for making it. Whilst rehab might be scary in some ways it's really easy because you have at least one thing in common with everyone else there!
One thing I would say is to have a good look at the sort of treatment the facility is offering, it's no good going to a 12-step place if such a program isn't for you etc (although I went to a 12-step place and although it's not my thing anymore I learned lots of good stuff there). Consider whether the living set up is something you can get along with. Whilst it's nice to stay in some plush hotel like facility, ask yourself whether that's really the right environment for learning how to live drug free in the real world. Ask about what sort of detox process they offer and consider whether you will be happy doing it.
Once you are there, just be prepared to engage fully. There's no point going and then holding back stuff that needs to come out. You don't have to disclose your darkest secrets on day 1, but it's probably a good idea to be prepared to cover them at some point.
Also, once you're through the detox rehab can actually be a pretty fun place to be if it's a good one. I enjoyed my stay (2 months) a lot, and that's not to say it was easy because every day was challenging, but I enjoyed it and made friends that I know I can call on at the drop of a hat for advice/a whinge/a cry/whatever without being judged.