Hi raecarter
Each Open Mic has a set of rules. If you have been to this one, you get some ideas. Otherweise, you can ask when you get there. A typical one is like "15 minutes or 3 songs", or on a busy night, "10 minutes or two songs". And that's the limit so you prepare one song to play, you can say "this is my first open mic ever so I just prepare on song for tonight" and there will be no problem.
I usually prepare a couple of songs and practice them more. It is up to you.
One etiquette - tune up before you go up. Usually the time is short, and you do not want to go on stage and start tuning. Find a quiet corner (or go to bathroom) and tune up. Be ready when your turn comes up.
But the Rule Number One is "have fun"
Cheers
Chief
PS: Where is the Open Mic? YOu're in Portsmouth (NH? RI? UK?)