Welcome to these boards! You'll find a wealth of information here about cruising.
You book a guarantee by deciding which category cabin you will be happy with and booking at that level. There's a chance you will be upgraded to a better category.
For instance, I couldn't stand an inside cabin
(claustrophobic), so if I were booking a guarantee I would book an outside (with a window) cabin guarantee. The worst cabin I would get would be an outside cabin anywhere on the ship. The best would probably be a free upgrade to a balcony cabin.
The guarantee cabins cost less than picking a specific cabin.
If you are using a Travel Agent this should have been explained to you. If not-get a new TA! Since this is your first cruise if I were you I would use a TA. Find one who is a "Cruise Specialist". It won't cost you anything (some charge a small research fee, $25.00 or less) and a good TA is invaluable!
BTW, I'm not a TA, I just have a great one!
Hope your first cruise is great! Princess is our favorite cruise line. (So far.)
If you have any other questions, just ask. You'll find that the cruisers here will be more than happy to help!