If you are new to kayaking comfort ought to be a big priority on outfitting any kayak. If you are not 100% completely comfortable in your boat you won't want to learn or be on the water. As a general rule your outfitting should not cut off you circulation or cause muscles to cramp or feet to go numb. As you progress in the sport you may want to trade some comfort for the performance abilities available in today's ultra small playboats. He are some rules of thumb:
a] Seat positioning. Start out with the seat in the middle and position the foot pegs or bulkhead or foam to allow your feet a snug fit without causing discomfort. After adjusting to this seat position (over many days of paddling) you may want to move the seat up and body weight forward to initiate more advanced moves... but only experience will decide that. Having the back band adjusted properly will probably be the most important thing for you initially.
b] Knee positioning: The knees are critical in whitewater kayaking. They allow you to lean and roll a kayak at will. They need to be positioned so that you can easily lean or rock a kayak back and forth from side to side. The knees should be positioned so that they are ALWAYS in contact with the top deck of the kayak and the pads. As a rule the knees keep the kayak from rotating around your lower torso. If they are too loose you will spin inside the kayak and miss a roll or turn because your knees slip instead of lever.
c] Hips and Seat: Hip pads are are also another critical junction where the body contacts the kayak. They should be tight enough so that you don't slip out of the kayak when you are upside down (making it impossible to roll) but not tight enough to hurt. A seat pad is critical to avoid sliding on the seat when trying to turn.
d] Tighteness: The tighter you are in your kayak the better - especially in class IV - V water where kayak response is critical. The idea is to make the kayak an extension of your own body, so that a slight movement from your muscles translates to movement in your kayak. This is the ideal, however the trade-off in a snug fit is discomfort. The idea is to get a snug fit as possible without causing pain - and also factoring in the water you are paddling. On a class II-III float trip you won't need to be making critical moves all day, so be in an ultra snug boat will be overkill and and agony.
Don't be afraid to sand cut or use duct tape inside your kayak to get the proper fit.
A good article can be found here:
ergonomics.about.com/od/ergonomicsatplay/a/wwkayakergo.htm