Is it better to sleep on your back, left side or right side and why?

Makes no difference: There is no "better" way to sleep for most people. What's important is that you're comfortable and sleep well. It may be better to put a pillow between your knees if sleep on side so that you're hips and back don't get twisted which could potentially lead to back pain.

Back: Most agree the back is best. You have less pressure on the limbs and hips and joints in this position. Back and neck are supported in a straight line, diaphram is not compressed, and no lateral stress on lungs or heart. Get your body in a relaxed position, proper head support, and give it a try. Good luck.