gymnastic rings aren't going to be such a miraculous cure for kyphosis, I'm afraid. They'll help, absolutely, but they won't be enough on their own.
You need to train scapular retraction (think horizontal pulling) and scapular upward rotation. The scapular upward rotation is a very overlooked component.
Those exercises LoRez linked for the rings are good. Buy some resistance bands too. Google the exercise "prone trap raises" and do those with your band. Do them every day! You can do them by getting down on all fours and putting on end of the band under your knee. You'll see what I mean after you've googled the exercise.
Another important upward rotator is your serratus anterior. Google "scap push ups" and do those every day too.
So to summarise:
-rows on the rings
-face pulls with the rings
-rear delt flyes with the rings
-prone trap raises with a band
-scap push ups
the problem you might have is not being strong enough to do the face pulls and rear felt flyes with the rings. You might want to consider joining a gym so you'd have access to cables.
Be patient. You're not going to fix 20+ years of postural deficits in a matter of weeks