There are several different kinds of posture for Zen meditation. You should choose the one best suited to you depending on legs and body structure. The postures are: full lotus, half lotus, high cushion, small wooden bench and backless chair. As much as possible, you should try to develop a full lotus posture with the left leg resting on the right thigh, and the right leg resting on the left thigh. But no matter which posture you use, there are important points to practice, which are the same for all postures:
Your Posture is a reflection of your attitude of mind. A posture which is too straight, with shoulders held back in military style of being "at attention" reflects a proud mind, which is an obstacle to meditation. One who sits proudly thinking "I am doing Zen meditation", is already wrong and is only adding to his ego through sitting. A posture which is too relaxed, with shoulders slouched over and with the back bent, reflects a lazy mind which is also a hindrance. It is difficult to maintain concentration for good meditation with an overly relaxed posture. Posture which is too straight or too relaxed also produces physical problems such as pain in the back of the legs or shoulders. The ideal posture is neither too straight nor too relaxed; it is soft and gentle and outwardly reflects a compassionate mind.
As much as possible, it is best to have an experienced person check your posture frequently in the beginning until you establish a good solid physical foundation for sitting. As long as you have correct posture some pain in the legs is not a problem and you shouldn't worry about it. Only if you are sitting incorrectly can it become troublesome. There is one serious problem that can arise from sitting incorrectly, which is pain in the spine. If you develop a pain in the spine that cannot be relieved by breaking posture, it is best to have someone with experience check your posture before continuing. In most cases, however, do not dwell during meditation on any aspect of your posture because this takes your mind away from correct meditation.