I have pain in my left hand, ring finger knuckle. i'm sure that some of it is related to playing guitar. suggestions?

Pain ring finger: hello ~ yes I suppose if you use a plektrum and balance your hand on the guitar with the ring finger, that could do it. But look carefully at everything to do to see where the overuse is. Piano, other musical instruments, holding a trumpet, gardening, computer work etc. It will get better. ONE thing you MUST NOT DO, is IMMOBILISE it or strap it because that causes fingers to go stiff very quickly.

Tendonitis?I'm a: guitar player since age 5.I have never had this problem but if you've been practicing for hours , days, weeks, you can cause inflammation .The cure is to first stop the activity that caused it.You may want an experienced instructor to look over your technique.Ice for 10 min every 4 hrs,Advil/Aleve as needed.You may need to avoid the activity for a few weeks to fully recover.Treat it like a sprain.

See advice: While it is possible you may have some arthritis in the joint it is equally, if not more likely, that you may be putting too much stress on the joint because of poor technique. Consider having a music specialist observe your positioning as you play to see if it's your form that is the problem.