Lonely up there all by yourself, isn't it, Skipy!

Use a guitar pitch-pipe to get you started in a good key, if you don't use an instrument.
Sing a song that you really believe in.
Practise at the volume you will use on-stage. This is important. On-stage you are louder, and need a different key.

You don't have to have a good voice to put on a good show. Did Johnny Cash have a good voice? How about Bob Dylan, or Willy Nelson? I've paid good money for their records, and I'd pay it to see their shows if they were local.