1. Right click on the 'My Computer' icon on your desktop and select 'Properties'.
2. Click the 'Advanced' tab.
3. Click the 'Environment Variables' button.
4. Click 'New'.
5. In the 'Variable name' field, enter 'JAVA_HOME'.
6. In the 'Variable value' field, enter the directory (including its full path) where you installed the JDK.
7. Restart the computer.

For your current user:

1. Open up a shell / terminal window
2. vi ~/.profile (replace vi with your favorite text editor)
3. Add export JAVA_HOME=/path/to/java/home/dir on its own line at the end of the file
4. Add export PATH=$JAVA_HOME/bin:$PATH on its own line immediately after
5. Save, and restart your shell
6. Running java -version should give you the desired results

For all users in the system:

1. Open up a shell / terminal window
2. vi /etc/profile (replace vi with your favorite text editor)
3. Add export JAVA_HOME=/path/to/java/home/dir on its own line at the end of the file
4. Add export PATH=$JAVA_HOME/bin:$PATH on its own line immediately after
5. Save, and restart your shell
6. Running java -version should give you the desired results

If you are using a GUI, you may not need to open up the shell. Instead, you might be able to open the file directly in a graphical text editor.