Choosing the right boat is a big decision. Consider all the options--hull types, including rotomold, kevlar, inflatable, etc. The type of paddling you'll be doing will influence your choice, of course. And if you're new to the sport, and afraid of being trapped in your kayak in a capsize, you might want to consider a sit-on-top model.
Jones has got the right idea here. It's smart to go into a store having done at least some research of what kind of kayak is right for the places you'd like to take it, for instance. If you'd like to rather kayak in the ocean than in a lake, a sea kayak (a sit-on-top model) is a better choice. Of course, there are people in the stores who are knowledgeable of the product and can help you get even more specific with your purchase, I'm sure. A kayak is a big ticket item, so if you're new to it, my best suggestion is rent one first, and if you like it, jump on in, the water's fine!