Open up Disk Utility (you can find it with Spotlight, the little blue magnifying glass in the top right of the screen) and in the sidebar select your thumb drive. Now go into the 'Erase' tab and select the file system you want to use (Mac OS Extended (Journaled) will work with only Macs, MS-DOS will work with Mac and PC) and type a name for it. Then click Erase and it will be formatted.
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