I would suggest starting from the bottom up. Get yourself acquainted with the
infrastrcuture before jumping into the business side of things. What kind of
database structure, site architecture, quota temples,etc. are in place. Putting
together a quality Visio doc always seemed to help me sort things out.
Once you've got that down, you can start looking at user demographics. Who are
your top users? What sort of active initiatives are in place? Sit down with
these users and identify current activities, pain points, goals, etc. and that
should paint a decent picture.