Secure the best spots sounds like FOMO to make people spend.
I'm going on a cruise on Tuesday. We haven't booked excursions, we've researched the places we're going and figured out what we would like to see under our own steam - in a couple of cases using local public transport.
I agree with AndyF. About the excursions.
Whether you go on excursions through the ship or do your own thing depends a lot on your previous travel experience. So if the cruise port was someplace I'd been before (not on a cruise) I wouldn't necessarily want to book a ship excursion.
Some cruise lines do not allow newbies to book excursions early. Disney gives precedence to people who have previously cruised with them. A first timer can't book early
On some cruise lines (like on some river cruises), you don't have to 'book' the excursions - they are already included in your fare. And you don't even have to decide which ones you want to take until the day before.
Assuming you are talking about a mass market ocean cruise, the tip I would give first timers is - check to be sure you have your passport before you leave. Actually that is a good tip for anyone setting out on a trip.
An actual first time cruiser tip - book the trip through an experienced travel agent who does a lot of cruise business. If there is a problem he or she will have a better chance of fixing the problem than if you book it yourself or through the cruise line.
I've removed the original post because it was just junk, but considering the question is still an interesting conversation point, I've left the thread up. Feel free to add your tips for first time cruisers if you have any. 
Quoting AndyF
Secure the best spots sounds like FOMO to make people spend.
I'm going on a cruise on Tuesday. We haven't booked excursions, we've researched the places we're going and figured out what we would like to see under our own steam - in a couple of cases using local public transport.
A lot of times in the Caribbean I will walk off the ship and go to the taxi rank and say, "I want to go to these three places (list them or have them written on a piece of paper). How much will it be to take me there?" and then we negotiate. These are places where the taxis don't have meters - it is a point to point fare depending on how many people are going - the fare is per person and the more people there are the less each one pays. I negotiate for a flat rate for the time.
I will occasionally use a free shuttle if one is available, but I rarely use public transportation in the Caribbean as it tends to be idiosyncratic - Some places have route buses where someone will decide they want to drive a bus route and will just do that. You really have to be local because the schedules aren't really published. In the USA, Canada and Europe I would be more apt to use a form of public transportation. New Orleans, NYC, Baltimore (where I grew up), D.C., Seattle, Toronto and in Europe I did use the water buses in Venice, and I have used the hop-on-hop-off buses several times in various places when traveling with grandchildren. The last time I used public transportation extensively was in 2002 in London and in Cheltenham.
Excursions usually are included on "expedition" voyages, especially to remote regions such as those in the Arctic and Antarctic. These excursions are among the best, with lectures by naturalists of what you are about to see and experience.
Yes - expeditions - also in the Galapagos
If you intend to fly back home from a port stop overseas, consider your boat could be behind schedule. (Weather, dock space still occupied, mechanical problem, etc.)
Perhaps it would be wise to not book a departing flight too close to your expected arrival time! Even spending a day in a hotel before flying back home could be helpful. Miss your flight home will probably be more expensive than a hotel room.
Happy Holidays!