What "details for preparation" are you looking for? What exactly are you planning to do?
There are many duties an Oracle DBA performs and therefore there are many reference manuals for the various duties.

If you're looking for a starter manual, aside from Oracle's own documentation, Oracle Press has many books that would get you started and then some. O'Reilly also has a ton of books on Oracle DBA tasks.

I would suggest that you sign up for two things, Metalink [url]http://metalink.oracle.com/[/url] (you need a CSI number)
and Oracle Technology Network [url]http://technet.oracle.com/[/url] (free)

These are good places to get various types of documentation on whatever you might need.

Good luck