Never forget that YOU are the best there is. They are very lucky to get to hear you. You are the greatest.

Ok, now that you are in the right frame of mind - go for it.

You might want to check out an open mike first before you go marching in there with your guitar in hand. They might not sing your kind of stuff. And one time I turned up for an open mike at a bar and no other performers showed up, so I had to sing for the whole evening.