I had good results with Bovada a few months ago when I played there, decent site and legit from all the threads about them that I've read on Reddit and elsewhere. Also available to Americans the last time I checked if that applies to you.
There's some negative feedback on the Bitcointalk account representing NitrogenSports at the moment (
click for link), and as far as I can tell most of the issues raised by the negative feedbacks have never really been fixed, so I can't recommend the site at this time. Used to be decent back when I did use the site though.
Bovada is probably the most popular and trusted poker site out there. They often have good welcome benefits + fast deposits and withdrawals. If you are nervous and aren't familiar with the poker industry: choose Bovada