I LOVE cruises. Before the kids were born we would go at least once a year. I love european cruises, but those are obviously more expensive with air and cruise prices. If you go in July or August, I would highly recommend Alaska. It is so beautiful! It is a little cooler, but if you are into sights more than beaches, that is a great option. Mexican Riviera cruises are great for beaches, and so are the Caribbean ones. Those you can pretty much do any time of year, except, as someone else mentioned, hurricane season (though tickets are cheaper during that time if you wanna take the risk, and if you do, definitely get travel insurance). Actually travel insurance is not a bad idea regardless of season :) ... anyway.. my favorite cruise line is Royal Caribbean. They also own Celebrity Cruises, though I've never been on one yet, but its supposed to be as good or better. If you like to party with the younger crowd though, carnival is probably a better option for your, though I feel they lack in the level of service that Royal Caribbean provides. I am 30 now, and was mostly in my 20s for all our cruises, and I still preferred Royal Caribbean.
I usually book through my travel agent, but only because my husband has a lot of food allergies, and she takes care of all that for us. For a typical booking, you might be fine going through Travelocity or Expedia, or the cruise line's website. Just browse around for deals.
As far as cabin choices, you will have your inside room, without windows, the ocean view with a small window or a large window, your balcony room, a suite ( a little bigger than balcony, also has a balcony), and other rooms I never looked into cause i can't afford those :) If you don't plan to spend much time in your room, get the inside cabin, its the cheapest. Or at most an ocean view one. The only time I got a balcony was for our honeymoon, cause I figured we'd be spending some time in the room, if you know what I mean ;) Once, I was upgraded to a suite when we arrived because there were some available. That was nice, the shower is bigger, which is huge, considering the size of showers in a normal cabin. I personally wouldn't spend extra money on that though, but if you have the money, go for it :)
Ok, I rambled on enough... If you have any questions, feel free to contact me. You are gonna love cruises!!!