Be sure to pick your foot all the way up so your heel is touching your yoga costume (if you wear longer shorts, imagine you are wearing a little tiny Speedo, or awesome tiger shorts like Orlo's- that's how high up your foot should be!)
At first, or for a while, you may have to hold your foot up with one hand so it does not slip. Your knee may have come up and forward a lot when you picked up your foot- now all you have to do is begin slowly pushing the knee down and back, so that eventually you have two knees in one line from the side.
Bring one hand up to the center of your chest, then the other. Don't discourage if your foot slips when you let go! Just keep holding it for now. When your ankles, knees and hip joints open more, the foot will more easily remain on your hip by itself.
This posture will help you prepare for the next posture, Toe Stand pose, by opening up the hips and strengthening your powers of concentration and balance. It also releases strengthens the internal oblique muscles which helps relieve back pain and prevent hernia.
One of the best ways to learn more about the postures is to read Bikram's "Beginning Yoga Class" book. Attend class 3 to 5 times weekly to put your learning into action and make your body smile!