Most kayaks will come with a thigh brace system. The thigh brace is there to help you control your kayak with your thighs.
When you first get into your kayak it is a good idea to have a look at the thigh braces and check that they fit your size. A lot of creek boats and white water kayaks have adjustable thigh braces. You may want to spend a bit of time adjusting them so they fit comfortably against your thighs. See the specific kayak manufacturers outfitting guide to find out how to adjust your kayaks thigh brace.