What is your itinerary and when are you going? These can make a difference in your experience.
Although I've cruised I have done so reluctantly and at the request of my spouse. My experiences have left me unimpressed with the entire process. So consider what follows as coming from a cruise cynic:
Upgrade to a balcony if you haven't already booked one. That little private space keeps me sane.
Be cautious about your consumption of food and alcohol particularly if you have an established tendency toward weight gain.
Patience is your friend. There are thousands of other passengers. There will be line-ups. Get used to it.
Go ashore every chance you get. Take the cruise company excursions if there is something specific you want to see or do. They are more expensive, you could probably see it on your own but for your first cruise it's a safer way to go. The ship will not leave if you are on a company excursion. Don't worry about anything they call "strenuous" unless you qualify for handicapped parking.
Remember that the cruise company's objective is to part you from your money while you are on board. Be wary of the shops. What seems like a good deal might turn out otherwise.
On your last day you will get a bill. It will be larger than you think. So will everyone else's.
Finally, keep an open mind. Cruising is not for everyone but it is generally a safe and convenient way to see different places that you might want to go back to someday.