I have osteoarthritis in my knees and shoulder. should i be taking a calcium supplement?

No: You don't need to take extra calcium if you have osteoarthritis of the joints. If ,however, there is some indication that you have some osteoporosis then calcium may be needed. The two conditions are completely different. There is some information that suggests glucosamine sulfate might be helpful (but this perspective is not shared by everybody ) . Your pharmacist can give you more information