You should talk over time of year you would like to get married and contact the resort for quotes. Make sure that they can accommodate the beach ceremony and reception and the estimated amount of people you are expecting. I recommend going through a good travel agent if you can (I used our beloved host Tammy) because they can sometimes negotiate better rates, extras etc. and know what kind of things to look for. Of course going through a bad travel agent probably won't get you anything but it probs won't hurt that much either
Make sure you look at menus etc. (even if they're the 2008 ones) so you can estimate general per person costs, etc.
If you know 100% that this is where you want your wedding then go ahead and book your date!
Once you get the date set then sign your contract (for the ceremony and room blocks if you're doing one) with the hotel and make sure you read through it carefully. If you've been promised anything (upgrades, discounts, transportation, etc.) make sure it's included in the contract.
Then you can start working on Save The Dates cards...