I'm not a seasoned cruise veteran but I've been on a few, so here's my advice:
pick and choose on-shore excursions carefully, as they can be expensive. Don't try to do everything.
as others have said, avoid the ship wi-fi (it is really criminally ridiculous the way the cruise industry rips you off in this area -- makes airline Wi-Fi look like the deal of the century!).
Don't fall for the pro photo ops unless you are genuinely a bad photog/have a crappy camera.
Bring something nice to wear for evening dinner.
Don't be afraid to make new friends! Lots of fun people on cruises, enjoy them while you're together and maybe some lasting friendships will be forged!
The ship will try to keep you busy all the time -- but you're on VACATION. Don't forget to relax and do nothing but eat some days!