Loads of info here
http://www.cruisecritic.com/ Browse the site for all aspects.
My tips from memories of my first cruise.
1) You do not have to do anything, just do what you want.
2) When you board, you will not get your cases until about 7pm or later, so take a small case with you as hand luggage with a change of clothes/swimwear or whatever. Assume you will not get your cases until 10pm!
3) Formal nights. I just wear a suit, not a DJ and my wife normally wears a skirt and blouse. If you have more than one formal night, you just need a different blouse for a 'new' look. Smart casual is one step down from formal. Most blokes wear a jacket, some wear a tie (I don't). Ladies just dress 'smartly'. Casual nights, blokes just need trousers and shirt, ladies almost anything!
4) Assuming you are on a big table(6+) you will meet friends no problem. If however you do not get on, speak a different language etc, do not hesitate to talk to the maitre d' who will put you on another table no problem.
5) (and most important) ENJOY YOURSELF!!!!!