Open launchpad, click the "Other" folder and select "Disk Utility" (in the red box in the attached image),
[image|2997768]
and choose your external drive from the sidebar. Then, from the top bar, select erase. It will pull up a window with numerous options. If you just plan to use it for storage, just leave everything at the defaults and click erase. A pop up will appear asking you if you are sure. Tell its to go ahead and erase. It'll load for a minute or two and then, Presto! Your disk is erased!