Enjoy It is the main thing
Cannot speak for the rally experience but have done similar on the track.
) Unless you are a seasoned pro and know it all then pay attention during the briefing sessions. They may be out of work racers and occasionally a bit patronising - but they have to cope with all abilities from moron upwards and are really trying to give you the info you need to enjoy the experience.
2) If you are driven by the instructor ask questions if the info you want is not forthcoming - again the more you know the better you will be able to handle it when your turn comes to drive.
3) Don't overdo it when it is your turn. Those who do and are all over the place will be black flagged and then it's no fun at all.
But over all ENJOY