How to reformat SD card?

I feel rather dumb asking this, but I just imported a full SD card into LR using a cardreader, and now I don't know how to empty-reformat it so I can use it again.

(I'm in France and don't have access to my iPhoto library, as it's at home on an external HD. Don't know if that's relevant...)

Do I need to put it in my camera to do this?

Thanks,
K
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http://www.pbase.com/karenmickleson