Get a data modeling tool, most have a way of connecting to a DB and reverse engineering it into a physical data model (showing tables, columns, table relationships, etc.). Here a couple of free ones which support all DBs and have reverse engineering capabilities:
I would get a data modeling tool and reverse-engineer my database first. Most decent modeling tools have a way to export the data dictionary to Excel or to a report. Oracle has a free one.
Oracle Data Modeller can reverse engineer and do this. It's free and although written by Oracle, does work on other RDBMS platforms (SQL Server, MySQL, etc).
If you want free then there is Oracle SQL developer data modeler.
It's good for beginners without too many features to get you confused but enough to get the job done.
Looks like they broke it out into it's own app now, sorry, haven't updated in a while. It used to be a plugin into SQL Developer itself.
http://www.oracle.com/technetwork/developer-tools/datamodeler/overview/index.html