What ship are you cruising on? There are some cruiselines out there where you can pre-book your excersions, I know Princess does that. If not, as soon as you board and after you find your cabin, you should check out the hours that the excersion desk is open and sign up as soon as you can. I know on Voyager and Explorer you can do it from your cabin through the TV, great feature.
As for boarding the ship, sounds like noon to 1:00 you should be able to board, check with your travel agent, they should know the soonest you can get on.
Have a great trip and hope you get all the excersions you picked :-)
I recommend getting to the pier as soon as possible after noon -- or even a little before noon if you are able. The worst case, if you are too early, is that you might have to wait in the terminal until the pursor's staff is ready to start embarkation.
As a practical matter boarding should be completed as soon as possible, and shore excursions booked as soon as possible. We have been shut out of some excursions as we were tardy getting to the excursion desk. Your local library or any of large chain bookstores will probably have a book or two on cruises and ports of call. These may be perused to get some advance conceptions of which excursions sound appealing to you.
As you board the ship, you will be entering a world unto itself. As a dyed-in-the-wool cruisaholic, I embrace that world away from the worries, cares and responsibilities that make up my everyday existence. Were it up to me I would walk onboard, get my room card directly from the previous occupant, have breakfast with the departing passengers and then get a chair somewhere where I can relax and watch everyone else board, or perhaps nap, or perhaps have some coffee with dessert, or maybe take a dip in the pool, or etc.