Sometimes you may find that your USB stick has a virus. In that case, you need to format it as quickly as possible in order to stop that virus from being spread to other computers.
To do so, plug the USB stick into a computer, (assuming you’ve already used the USB stick on that machine, in which case you’ll likely have to scan that for viruses as well), and then head to the My Computer page.
Then, right-click the stick, and then click on “format”. You’ll be given prompts on what details you want, including the capacity, the file system, and whether you want to use a “quick format” or not.
Just follow the prompts, and then you’re ready to format your stick. But beware – back up everything on there before you do.