How to fix the pain in my shoulder that I bow the cello with?

Chiropractor

You don't give us much information, like your age, how long it's hurt you, and so on. So it's a shot in the dark. If this has been going on for a long time and is stopping you from doing your normal activities, see your doctor. Especially true if you're older or have played a lot of sports. OTOH, if it's a recent pain and not too debilitating, use ice packs (ten minutes on and ten off) and rest it as much as you can for a few days. Also, don't sleep on that shoulder.