To do the Sit Up, begin lying on your back. Raise your arms so your biceps are next to your ears and cross only your thumbs with your palms facing the ceiling. Engage the fronts of your thighs and shins to flex your feet like you're standing on the back wall, toes toward your face. Keep your arms and head together, your heels on the floor.
Take a long, deep inhale as you begin and hold the breath in until you get to the forward bending part. Keep your chin toward your chest as you sit up, keeping your arms and head together and your heels on floor. The Sit Up (until you get your head to your knees the first time) should be one continuous motion.
When you're up and on your way forward, reach out to grab the big toes with your middle and index fingers, exhaling strongly once by pulling your abs in as you touch forehead to your knees. That first exhale will cause you to "bounce" up away from the knees- so exhale the same way another time and touch your forehead again to complete the double exhale. The more powerful your exhales, the more energy you will have for your next posture.
This is the best (and cutest) visual demonstration we've found, from yoga champion Joseph Encinia.
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!