Check your pills:
www.dancesafe.org
www.ecstasydata.org
www.pillreports.com

Real ecstasy is hard not very easy to overdose from and could take up to 8 good quality pills. If it is your first time one pill may be enough, your body weight may effect how much you need.
Ecstasy has always made me feel completely free, almost as if I am not human and I usually just want to talk or lay on the grass and look at the sky.
Music is a must, don't worry about hydration so much unless you plan on doing something physically active like dancing in which case you would be sweating a lot but it is true you should have some kind of drink, sports drinks are best (Powerade, Gatorade) as they contain electrolytes which your body also looses when you sweat.

Read some of the experiences on erowid so you know a bit about what to expect, but when you're rolling let the mdma take you where it wants you to go, trying to control it yourself could bring you down.