This walks you through the steps of automatically backing up your Mac to an external hard drive using free software. In this case, the only piece of software you need that is not already on your Mac is RsyncX.

Sure, Macs come preinstalled with Unix rsync, but RsyncX is needed to preserve Mac-specific resource forks, in case you have any apps that still use them. It also provides a nice GUI that is probably easier for most to use than trying to remember unix commands.