When you sit to meditate, sit comfortably, with your spine reasonably straight. This allows the spiritual energy to flow freely up the spine, which is an important aspect of meditation. Leaning against a chair back, a wall, headboard, etc. is perfectly all right. If, for physical reasons, you can't sit up, lay flat on your back. Place your hands in any position that is comfortable.
Any form is all right.
This can be quite helpful, but is not absolutely necessary. Meditation is turning off your thoughts or making your mind a blank. Inner quietness is experienced in meditation, but not by willfully turning off thoughts. Meditation is difficult and takes tremendous discipline. Meditation is only difficult if one tries to do it perfectly, which is not really possible.