Most of us have been there: You're out taking photos and your memory card is full, but you don't have another with you. Or ...

The Computer interface opens, listing the SD card as a removable drive. Right-click on the SD card entry in the Computer interface and choose "Format" in the context menu.

The best way to clear an SD card is to format it, which is like hitting a factory reset button; it'll erase every file, leaving just an empty card.