Welcome to the world of cruising!
My biggest tip would be to just relax and enjoy it! You can do as much or as little as you want on a cruise and have a great time either way. With only 3 nights you wont get to do it all, but don't be disappointed by that. Just enjoy what you DO get to do.
Also, be warned...these cruises can be addicting. My first cruise was in 1995 for my honeymoon...and I didn't think I would like cruising. 33 cruises later (29 of them Disney), I guess I thought wrong.
If you have any specific questions feel free to ask. You will get a ton of info here!