1) HAVE FUN
2) Try to take advantage of as many of the activities as you can (they're great!), but don't stress out trying to do it ALL!
3) Spend some time looking over your Personal Navigator (the cruise "itinerary") and plan "Must-Do"s and maybe a few "if we're not too tired".
4) Explore the boat on the first day so you get a feel for where everything is... try out the different decks.
5) Be social and meet people-- most people on the Disney cruise are there to have a great time... try not to take yourself too seriously. And don't get grumpy in the long lines for character photo ops.
ENJOY!