Singing Karaoke is easy - The hardest part is taking the first step. Bring some friends, or come early, when there are not so many listeners.
Have a look at the songbook, and pick a song. Start with a song you know well - or - Pick an artist you know, who's voice match yours. Don't pick a group, pick a single artist. If you are able to sing-a-long to all/most of this artists songs, without straining your voice, this is a good place to start.
Remember: The first time on stage is about doing it - Not how you do it.
Give the song-number(not the name) to the Bartender or the KJ.
Sit down and enjoy the Show, and wait for your song.
20 seconds before your song comes up, "PREPARE FOR: xxxxx xxxxx(song title)" will be displayed on the screen.
Get on stage, and - Kick A**
Listen to the music - not your own voice. The sound on stage is very different from the sound the audience hears.
Don't get frightened hearing your voice, and move away from the microphone. Focus on the music, and the rythm.