You should experiment with different postures to find ones that work for you. And the posture you use might be different depending on where you are and the practice you are doing. For instance, when I am with a group, I meditate sitting in a chair or on a cushion, but at home I meditate lying on my back.

In general, you should have a position that allows your spine to be straight and your body well-supported. That means, for instance, that you should not be wobbling back and forth (or be prone to wobbling if you become drowsy), and if you are sitting in an Eastern style position (lotus, half-lotus, Burmese, etc.), your knees should be firmly planted on the ground/cushion, not floating in the air.

You should also choose a posture that allows you to be comfortable, so you are not excessively focused on the physical body during your practice, but not so comfortable that you fall completely asleep. At the beginning, you will probably have to choose postures that keep you more alert, such as sitting upright. Once you develop concentration and can maintain the continuity of your consciousness even if your body is asleep, you can shift to more relaxed positions.

Study the section of the Gnostic Meditation course on posture for some more tips (link is below).