The first time you make the decision to go to a karaoke bar there will be several rules for etiquette. Depending on where you go, the tips could vary. No matter where you go, my first suggestion is to get there a few minutes before they begin. This will give you enough time to grab a drink and watch what goes on while you get up your nerve to sing.
TIP #1: Do not boo or heckle anyone on the stage! - Keep in mind that this is NOT American Idol and some may be really good while others may butcher your favorite song. Everyone goes to karaoke to have fun and unwind, not to worry what someone else thinks about their singing ability.
TIP #2: Give applause for everyone! - Even if it's the worst rendition of "I Will Survive" that you have ever heard, clap anyway. Others will return the favor no matter how bad you suck! Karaoke singers seem to have camaraderie between them.
TIP #3: Do not select a song that someone else has already sung. - It's tacky and it can be viewed as a "challenge". It's not worth it. However, if it's the only song you know you can sing, it will probably be ok to sing it again. If you've given your request to the KJ (karaoke jockey), and someone else comes up with your song first, try to find another one. A variety of songs at a karaoke session is always good.
TIP #4: Keep to your skill level! - If you struggle with the words to a song or are unsure of some phrases, DON'T grab the nearest wireless microphone and start skipping around the room like Cyndi Lauper if you can't get the words right. Reserve this for the more experienced karaoke singers.