Have you tried the Oracle website?
THe data dictionary is a vast thing to mess around with.. generally there are a number of tables you'll find useful,
(then replace user with all_etc)
A good source would be Oracle DBA books by Osborne (publishers), they're expensive so join the local library.
I'm afraid it took me the best part of a degree to work out how to use Oracle and the Data Dictionary effectively so be prepared to spend some time on this one!!
Don't think, do!