You say you aren't a great musician. Two responses spring to mind.

1. Woodie Guthrie wasn't a great guitarist, and in fact many top class entertainers aren't. But they are entertainers, so they manage to engage an audience to the point that only the purist will actually notice their playing.

2. The way to improve is to find venues where there are musicians much better than you.

Join in and learn to do what they do. You will be surprised how much and how quickly you improve.

But please, please, please ask them first and explain what you are trying to do, and join in very, very quietly until you are sure you have the right key and know what version of the tune is being played.

Do that and they will be happy to have you along, and you will learn a lot.

Don T.