1. First and foremost figure out what type of cruise would best suit you. Large ships with lots of activities and dining options where the vessel is as much the attraction as the ports; smaller boutique ships that can provide a more up-close-and-personal experience by getting into remote places; a river cruise where you hop off each day to wander European cities or even drive it yourself up the Murray...
2. Find out what is included. Meals will always be included but some packages throw in your drinks and excursions in the main price, others charge them as extras. Most cruise lines have a daily gratuity fee added to your room for service. Be aware of this and check to see if it's already included in your total cost, or will be added daily once onboard so there are no surprises.
3. Do a bit research on the ports you are visiting before you travel. If you book the ship's shore excursions you will be well taken care of, but they can be more expensive than pottering off and doing your own thing. You may find there's shopping and outdoor dining (with free WiFi!) and is all you fancy. It is also a good idea to book your shore excursions before you go so you don't miss out. You can usually do this on the ship's website.
4. Take a smaller cabin bag or soft weekend bag as you will have your main luggage collected the night before you disembark so will need this to put your jarmies and toiletries in the next morning.
5. And finally, a warning: If you're going to Venice (and possibly other Italian ports), DO NOT purchase a knock off handbag! The police prosecute buyers not sellers and you could be pinged for €350 and marched to la stazione di Polizia. Mind you, it would make a great dinner party story when you get home.