Can you reverse engineer it into Erwin or some modeling tool? That would be my first try.
If not, draw your own diagram of each table and foreign key relationship on paper.
Also, you could use a profiling tool to look at the domain of each column. That would help you to understand what data is in each column.
What exactly do you mean by "learn" a new database? Will you be supporting it from a DBA, Application Developer, BA, etc. role?
-------------------------
The trouble with doing something right the first time is that nobody appreciates how difficult it was - Steven Wright