Learning to play guitarr

My niece came to me a few months with a classical guitar and told me that she was learning to play. I never had a big interest in playing guitar as a teenager but I had a few organ lessons (that my dad did me the favor of pay). So when I listen to my niece playing I thought, well, there's a guitar in the loft that is already there for over 30 years. Maybe it's time to get if out of the closet. And so I did. It took new strings and the thing to tighten the strings (do not know the name), it has been tunned and then started training a few chords. To start I learned the chords of "Dunas" of GNR. Next I bought a tuner and started to learn the music of REM "Everybody Hurts." I know the chords but it is the difficult to change quickly the chords and I can't play more than 30 minutes because the fingers start to hurt. But I still have hope of doing a duet with my niece. A few more workouts and get there.