Prerequisites for Learning Oracle SQL and PL/SQL

You can find the download links for various versions of the Oracle databases under
http://www.oracle.com/technetwork/database/enterprise-edition/downloads/index.html

Download your favorite flavor, and if you have any problems in installation then Oracle provides Installation Guides for different operating systems.

For Example (for Oracle 11g Release2) :

For Linux : http://docs.oracle.com/cd/E11882_01/install.112/e47689.pdf
For Microsoft Windows : http://docs.oracle.com/cd/E11882_01/install.112/e47798.pdf

You can also go for the Express Edition if you want
http://www.oracle.com/technetwork/database/database-technologies/express-edition/downloads/index.html

For development purposes we will use SQL *Plus and SQL Developer, both comes with the default installation package. Though most recent versions of the client tools can be downloaded separately.
Like: http://www.oracle.com/technetwork/developer-tools/sql-developer/downloads/index.html

Note: Considering the fact that JRE is pre-installed. SQL Developer needs the prior installation of JDK 7.