Welcome to Cruise Critic. And, more importantly, welcome to Cruising! Planning your first cruise can be a daunting task. Since this is your first one, using a travel agent might be the best route. He/she can make recommendations that could take you hours to research yourself. As far as prices go, most Travel Agents won't be able to get you a better price, but you'll likely receive an on-board credit or a gift.
The general rule of thumb is book early.... the prices tend to be the lowest the farther away the cruise is in time.
Also, keep reading these boards. I learned more about cruising reading and posting here than I did on actual cruises!
Once you have chosen your cruise, go to that cruiseline's forum for some real insider tips... Good luck!