Do you have a kayaking body?
You often hear people say that a kayak should fit, "like a glove" or "like a lawn chair". How about the best of both worlds - "like a comfortable glove"? That's how a kayak should fit under ideal circumstances. But everyone's body is different, so the kayak that fits one person perfectly may be unbearably tight for another.
When you're starting out kayaking you want to find a boat that fits snugly enough in the hips, knees and legs that you can easily control it by rocking your hips back and forth. On the other hand, the boat needs to be comfortable enough that you can sit in it for several hours without pain or discomfort. The cockpit should be large and unobstructed enough that you can easily get in and out of the boat and the boat should be forgiving - it should have edges that are well out of the water much of the time and that won't surprise you by grabbing suddenly if your lean isn't all it should be at first.
The whitewater kayak industry has evolved tremendously over the last 15 years so that there is now a wide variety of boats available to fit paddlers from 50 to 300 pounds and from 4' 6" to 6' 8". These advances in design have also extended to ease of control, so that the outfitting (seats, knee pads, foot systems, etc) and hull shapes make moves which used to be very difficult to learn much more accessible to beginner kayakers.
You need a moderate level of fitness to enjoy whitewater kayaking. You should be able to walk a few miles at a brisk pace, do 5 to 10 situps, lift 30 pounds or so and be active for 6 to 8 hours at a time. It doesn't take tremendous upper body strength to kayak, so don't worry if your biceps don't rival a body builder's. In fact, technique routinely outperforms strength on the river. There are excellent kayakers of all shapes, sizes and ages!