There are several seated meditation postures. The most well known is the full lotus, sitting cross-legged, with the top of each foot resting on the opposite thigh. Beginners often use the half lotus, where one heel is pulled up to the crotch, and the other foot rests on the opposite thigh. Also acceptable, however, is to straddle a cushion that will hold one in kneeling position, or to sit on a small bench, with the legs in kneeling position, or to sit on a plain wooden chair as you normally would.
One thing that remains constant throughout all the seated poses is that the back is straight, the hands and arms relaxed, shoulders wide and chest uplifted for easy breathing.