I went on one about a month ago, it was just a short one from seattle to vancouver, BC
It was on Princess.
advice? be hungry. The food is free (well, not really, but just pretend you're paying for the food in the ticket cost)
it's pretty laid back, be sure to be aware of the schedule of things going on during the cruise, the shows, the few hours that the restaurants are open, and so on. They have all hours/all you can eat buffets on the ship too, but the actual resturaunts just taste better.
you dont get your own table when you go to eat either, they'll sit 6 to a table (on the cruise I was on anyway) so you will get to rub elbows with people you don't know. (our table was mostly asian, they didn't speak much in the way of english from what I could tell so we just chatted amongst ourselves)
just try not to burn yourself out on the 1st day. The scenery on a ship doesnt change from day to day. I'm not really an easily amused person so one day was enough, not sure if I could have tolerated a longer cruise without another form of entertainment.
The ship we were on had a nightclub up on the 17th deck too, was pretty decent
all in all, have your papers in order and keep the lines moving.
oh and, you dont have to tip for drinks, the waitstaff on our ship had an automatic 15% charge as well as a 10$ a day fee that they divide amongst all the staff when their shifts are done for (I'm assuming when the cruise reaches port)