Osteoarthritis means degeneration of joint cartilage generally age related. 14-19 is not common age for Osteoarthritis. Kindly share how you were diagnosed. I suspect you may have Juvenile Idiopathic Arthritis.
Gluten allergy and inability to gain weight maybe an underlying Malabsorption Syndrome. Kindly see a Gastroenterology and get tested properly.
Glucosamine and Chondroitin Sulphate tablets in some studies have no advantages over placebo.
Share your Shoulder and Knee Xray.
Next Steps
Rule out Inflammatory cause of Arthritis, MRI of Affected Joint SOS.
Health Tips
Don't Google symptoms and medications. Consult a qualified doctor.