It's very easy:
Go to Applications --> Utilities --> Disk Utility
In Disk Utility, click on the volume in the left pane.
Click on the Erase tab at the top of this window.
In the Format drop down menu, choose Mac OS Extended (Journaled)
If you want to give this backup drive a more descriptive name, type that in the field below.
It should only take a minute or two at the most to format your drive.
(BTW, for easier access later on, I keep Disk Utility available in my Dock...saves time when I need to use it).