Watch the Follow the Rules of the Jam online guitar lesson by Jeff Scheetz from Blues Jam Survival Guide
Each jam night is run a little different. It is good to know the "pace" of the jam night - if it goes really fast, or if it is more casual. The best thing you can do to learn this is to go out to the jam night for a couple of weeks BEFORE you even take your guitar with you. Just go there to check it out and see how it all flows. This will also let you know if you need to bring your own instrument or use theirs, also see what songs would be good to play.