Jam sessions, practice sessions and similar terms tend to get used interchangeably despite having significantly different meanings. Just because someone says there is going to be a jam session doesn't mean that is precisely what they mean. Be sure to check that you are not supposed to prepare or learn songs beforehand. Normally this is not necessary for a jam session, but it is not unusual for someone to use the term when they meant a practice session. Practices are more focused on polishing known songs, and are unproductive if people go into them without knowing the songs beforehand.
At the beginning of the jam session, it is a good idea to start with simpler chord progressions and rhythms. It helps the musicians involved get into the same groove. This helps keep anyone from being thrown when more complex musical elements start getting used. Straight eighth-note rhythms tend to be a good place for most jamming to start. Melody players, such as lead guitarists, can start a little more complex.
Writing new material tends to be a common goal of many musicians at jam sessions. These sessions can be a good way to write riffs that would not normally be as obvious. Always have something to write down any good material written during the session. A high tech solution would be to use a laptop with programs like Guitar Pro or Power Tab to write out the score. A pencil and paper will be more than enough.
Many musicians simply record their practices and jam sessions. This is very easy to do, but ultimately far more time-consuming and wasteful. Anyone using this method will spend a huge amount of time just trying to find anything in particular on the recording. Then that musician has to figure out how the part was played. This often requires listening to the recording multiple times. Spending the little extra bit of time to write out good riffs saves all of this hassle.