"Most importantly - smile, have fun!"
Seconded - if you enjoy the song, it's a safe bet that the audience will too - so aim to please yourself.
Remember - the audience is on your side from the word 'off' and wants to enjoy your song.
"It's ok to have a cheat sheet"
Don't agree at all - it makes you look as if you don't know the song.
If you have no confidence that you know a song, choose one that you do; if you do dry up in a song it's odds on that the audience will help you out, or else understand the situation and wait till you get it together.
Jim Carroll