Time Machine is the fabulous backup application that’s part of Mac OS X Leopard. 
To use Time Machine, simply connect an external hard drive to your computer.

1) Choose "System Preferences" from the Apple menu.

2) Click the "Time Machine" Icon.

3) Click "Choose Backup Disk".


Time Machine toggles on, changes the image for your backup disk, and indicates when the next (in this case, the first) backup will occur). For best results, you should use a drive that’s at least as large as your Mac startup drive. And it’s a good idea not to store any other data on your Time Machine drive. That way, you maximize the amount of space you have to backup your files.