The preferable way to install java is to use your software installer, which is usually under menu/administration heading, but varies depending on which distro you are using, so it would help if we knew what distro you have. To install java manually open a root terminal to become root user from your main menu or open a terminal window from the menu and type either "su" or "sudo" without the quotation marks to become root user, once again it depends on what distro you are using. Will ask for password to proceed. Once you get a root terminal open then type the following
cp /<path to java file>/jre-6u7-linux-i586.bin /usr/
ln -sf /usr/jre1.6.0_12/bin/java /usr/bin/java
just remember version of java has changed.
"cd" is to change directories and "ls" is to view the files in the current directory. The first line above copies the java file to the /usr directory.