take a look at www.oracle.com
and switch to technology network. Here you'll find the actual version
of oracle for all supported platforms. When you start downloading documentation or software you'll be
prompted for your username and password. New members get a choice to join the otn (oracle technology
network). Once you have registered you can download the software and docs you want. However, take a
close look at the license!
Do not simply try to install - go through the manuals first. Installing a db like oracle is a complicated
process. The manuas give you a basic idea of the required system configuration required to run the db.