If you can`t sing, you are not drunk enough... ;0)

No, seriously: maybe you haven`t found the right songs yet. It won`t sound great when somebody with an untrained voice is trying to do a song that even good singers would find difficult. But since you like folk you won`t have too much trouble finding songs that are easy enough. Sometimes accompanying yourself on an instrument helps to stay in tune (or reasonably close to it ;0)

And what the others said.

Good luck
Ernest (same situation here)