I think it's awesome that you are looking to get out there and play for people.

The only thing I would add to the great advice you got already is to go check the open mic out first just as an audience member. Bring your stuff *just in case*, but go with the midset of just checking it out. Getting up there can be nerve wracking enough that you don't want to pile the unknowns of the open mic on top of it.

If you plan on doing a solo thing, having 3 or 4 songs ready to go is perfect. If you plan on sitting in with the house band or other players there, you should have at least 10 commonly known songs from a variety of artists ready to go. My first open mic I went to I was set to play some Doobie Brothers, but the singer we recruited said the songs were out of his range. So we fell back to 2 other songs he could do.

The most important thing to do is...HAVE FUN!! Generally the people there will be very supportive and appreciate the courage it takes to get up there.