I've been on one cruise and I'm going on another in 59 sleeps! Yay! I went with P+O last time with a good friend who has been on 22 cruises so far. She and her hubby love it and find it the perfect way to holiday. For tips I'll give you what I can think of from the top of my head and think about it a little more.
1) Your meals are included at the restaurants and cafes but some of the P+O ships have one or two restaurants which are a bit fancier. They may be started up by Luke Mangan or a similar "star" chef and are part of his organisation. These restaurants you will pay a small surcharge to go to, perhaps $20 a head, but it's a bit fancier than the restaurants in that they are more like what you'd expect from a fine dining establishment. The main a la carte restaurant is fine. Lovely. But the restaurant you pay a surcharge for is what you'd expect to see from a celebrity chef's restaurant. I don't know if I'm expressing myself properly but I hope you get the drift. If you want to dine at the surcharging restaurant BOOK EARLY... as soon as you get on the ship. Phone them up and book a night and a time... some people even book a table every night so you have to get in early.
The main restaurant on the ship is a la carte and very nice. The buffet restaurant is like a club bistro where you go and help yourself to whatever you want. The quality is probably a little better and the choices are more varied. Try all of them and see which you prefer.
2) You will get a schedule on your door sometime in the afternoon for the next day's activities. We went through it with a fine tooth comb and planned our day. There are so many activities on the ship you'll miss out on things you might want to do if you don't plan your days. Mind you, if you don't want to do anything that's fine too

There are trivia contests with teeny prizes such as P+O bottle openers. I won a few of them for my team. After only 2 days every time I walked in for the trivia comps there were whispers "oh see her, she's really good.." I wonder where I learnt so much..

There's also bingo, a live show most evenings, singers and bands in the lounges/bars, an art auction (bring your credit card...), a gym, a spa.. you can fill your days up or do absolutely nothing. The choice is yours.
3) The internet on the ship is really expensive so be prepared to do without. Prices change but on one cruise line it was $55 for 100 minutes or so and I think it's even more expensive on another line.
4) The staff are lovely and seem happy to anything for you to make sure you're happy.
5) The beds and pillows were so comfortable. I took my own pillow but didn't need to use it as the pillows were perfect for me.

I take my own pillow everywhere I go otherwise I get a sore neck which builds into a cracker headache eventually.
6) On a 7 night cruise there will probably be 2 or perhaps 3 formal nights in the main restaurant when people get dressed up to the nines for dinner. On the P+O (Australia) ships it seems to be a bit more casual than other lines but you will still see women in long dresses with loads of diamantes and jewellery on, and the men in suits. You don't have to do this of course. You can get dinner in your cabin, go to the buffet restaurant or do nothing but if you feel like getting dolled up, do so
7) Get international travel insurance and extend it to the day after you return. Mate of mine was on a cruise that was delayed due to engine trouble and they got into port 12 hours late. No a problem for her but for people who were catching connecting flights it was a massive problem. Because their insurance only went to that day and they hadn't contacted their insurer before landing they lost out on all the money for their flights and had to pay for hotel accommodation themselves. If they had got the insurance to cover them until the day AFTER their cruise they would have been fine.
8) Get your insurance the day you can no longer get a sizable refund for your cruise. My friend who's been on umpteen cruises just got back from a cruise (again) but it looked like she wasn't going to go this time as she broke her foot in 10 places 3 weeks before the cruise. Thankfully they had insurance so she and hubby would have been completely refunded but it's a word to the wise.. get insurance! A friend of mine has worked as a nurse on cruise ships in the Caribbean and you wouldn't believe the things that happen on cruise liners that people need medical attention for! And you'll need international insurance of course. Even on a domestic cruise which doesn't touch land in another country you need international insurance because getting a chopper to rescue someone on a ship is an expensive business and the bill would be horrendous.
9) It depends on the cruise line as to how the medical facilities charge patients but if you need to see a doctor or nurse it won't be cheap (insurance again..). Take whatever medications with you that you think you may need. You normally have a 2 suitcase limit per person so you can easily take loads with you.
10) Check with P+O but I think you can't take duty free alcohol with you. I have heard (not experienced) that they will take it from you and give it back to you at the end of the cruise so you buy from their bars and restaurants.
11) As you're walking up the gangplank there will be a photographer. I think he takes a pic of everyone for their records while on board so they have a true record of who's on board. Just grin and bear it.. they WILL take the photo so you might as well smile and get it over and done with.
12) They ask you to pack your suitcases the night before and have them ready by a certain time (about 6pm I think). You put them outside your room and they will be collected. The next day your suitcases will be taken of the ship and ready for you just as if you'd got off a plane and were waiting at the carousel. I'm guessing that this ensures there aren't hundreds/thousands of people wandering around with suitcases cluttering up the aisles/restaurants/lifts etc in the morning before disembarkation. That would be a safety hazard. It does mean tho that you may want to take a small bag with you. That way you can put those things you used overnight or need in the morning in the bag and take it with you when you leave. These may include nightclothes, the clothes you wore the night before, cosmetics, toiletries, pills etc etc.
13) Tragically for this tea/coffee addict there is no kettle in the cabin unless you get a suite and I'm not totally sure they have one either. I'm guessing this is for safety as they don't want people splashing boiling hot water on themselves if the ship is rocking. If you think you're going to use the stewards to bring you pots of tea or coffee (that you pay for) then you can tip them when you get on and then again before you get off. This makes you known to them and you're more likely to get faster service. I only got a pot of tea once so I can't tell you what the service is like really. There is always tea and coffee available in the buffet restaurant as far as I know.. but I can't promise.
14) Before you set sail you WILL go to the talk on emergency drill. I gather it's international law that every passenger (and prob crew member) attends and they will insist you go. You take your life jacket with you so you know how to put it on just in case. It takes maybe 30 minutes and after that the ship is yours
15) My friends like getting an inside cabin. That way the light in the morning doesn't wake them and they sleep better. I think it would be a bit cave like to not have natural sunlight but you're not in your cabin that much apart from sleeping, showering and dressing. Inside cabins are cheaper too.
16) The higher decks get to embark first (that's the plan anyway) so if you want to get on earlier and get off earlier get a higher up deck. Disembarkation takes place in order of decks with the higher up decks getting off first. You don't have to get off early but if you have planes to catch then it's a handy thing. If you don't have anywhere to go in a hurry then you can stay on, have breakfast, enjoy the ship a little longer until you need to disembark. Disembarkation is done in groups rather than a come as you go procedure. You'll sit in a congregation area (a bar probably) and then that group will get off together.
17) If you like good food and think you might indulge a little more than normal, take loose fitting clothes. People have been known to walk on with nicely fitted clothes on walk off with tight clothes!
18) If I think of anything else I'll let you know.. or feel free to send me a note. If I don't know then I'll ask my friend who's been on 20+ cruises

Have a wonderful time and relax!