format

Time Machine is not a physical drive. Do you mean Time Capsule? Or do you mean formatting a hard drive you use for your Time Machine backup?

You format a Time Capsule using Airport Utility. Open Airport Utility, select Airport Utility Help from the Help menu and search for "erase."

If you have just a backup drive to erase:

Drive Erase for Snow Leopard and Earlier

1. Open Disk Utility in your Utilities folder.

2. After DU loads select the volume you wish to format from the left side list. Click on the Erase tab in the DU main window.

3. Set the format type to Mac OS Extended (Journaled.) Click on the Erase button and wait until the process has completed.