The easiest way to make backups of a Mac is through Time Machine > http://support.apple.com/kb/HT1427
To use Time Machine, open System Preferences > Time Machine > Select Disk, and choose your external drive. The initial backup will start, and it may take several hours, depending on the amount of data Time Machine has to copy to your external drive.
Since then, Time Machine will make automatic, hourly backups, always that your Mac is turned on and the external drive connected. Also, if you are using OS X Lion, local snapshots will be enabled > http://pondini.org/TM/30.html