Zazen or sitting meditation is the primary focus of Soto Zen and the Iowa City Zen Center. Please watch this excellent video on zazen (sitting meditation) Do not worry if you cannot sit in lotus position, as noted in the video you can sit on a chair or other positions that you find more comfortable on cushions on the floor. Arrive ten minutes early to get basic zazen instruction.
The 13th century Japanese monk, Eihei Dogen, the founder of Soto Zen, gives instructions for posture in zazen, in his Fukanzazengi, "A Universal Recommendation for True Zazen"
"For sanzen (zazen), a quiet room is suitable. Eat and drink moderately. Cast aside all involvements and cease all affairs. Do not think good or bad. Do not administer pros and cons. Cease all the movements of the conscious mind, the gauging of all thoughts and views. Have no designs on becoming a Buddha. Sanzen has nothing whatever to do with sitting or lying down.
At the site of your regular sitting, spread out thick matting and place a cushion above it. Sit either in the full-lotus or half-lotus position. In the full-lotus position, you first place your right foot on your left thigh and your left foot on your right thigh. In the half-lotus, you simply press your left foot against your right thigh. You should have your robes and belt loosely bound and arranged in order. Then place your right hand on your left leg and your left palm (facing upwards) on your right palm, thumb-tips touching. Thus sit upright in correct bodily posture, neither inclining to the left nor to the right, neither leaning forward nor backward. Be sure your ears are on a plane with your shoulders and your nose in line with your navel. Place your tongue against the front roof of your mouth, with teeth and lips both shut. Your eyes should always remain open, and you should breathe gently through your nose.
Once you have adjusted your posture, take a deep breath, inhale and exhale, rock your body right and left and settle into a steady, immobile sitting position. Think not-thinking. How do you think not-thinking? Non-thinking. This in itself is the essential art of zazen."