Assuming you want to save your files, like pictures and music, you will need to purchase an adapter for your pc if it doesn't have a multi-card reader built in.
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Plug the card into the adapter, then plug the adapter into your PC.
Open computer and save all the files you want to keep as you will lose them otherwise.
Next, left click ONCE on the card to highlight it in the main computer view where it shows you your drives and storage spaces. Then right click and select format. Click restore device defaults, and click "quick format." Then hit start format, it should only take a few seconds.
Once the format is complete, you can move the files you kept back onto the card.
On the taskbar, there should be a small icon by the system clock that looks like a usb stick with a checkmark on it. Left click it and select the eject option. If you don't do this, eventually you will end up with data corruption and need to buy a new memory card.
Now you can unplug it from your PC, remove the card, and put it in your droid.