So, you insert the memory card into your reader, and it demands that you format the disk before use?
Did you format the disk PRIOR to using it to begin with? Or, did you plug the disk directly into a separate device (camera, cell phone, portable gaming system, etc) without formatting it?
Some of the devices CAN format your cards prior to using them initially, however, that won't always ensure they'll work.
Do you have another reader you can try using, to rule out your existing card reader? If not, pick up an inexpensive one at your local electronics store (or mega-mart, for that matter). Should be able to find one for under $20, keep the receipt for future returns (maybe).
If you plug the card into another reader and it works, then your original card reader is to blame.
If you plug the card into a new reader and it still requests you format the card, I pray you don't have anything too valuable on the card at the moment, because you'll have no choice but to format it.
There is the possibility you got a defective card. If that's the case, the only remedy is replacement. This will be determined once you format your original card. If it still prompts you to format, your card is defective.
If you need clarification on any part of that, let me know. I kinda got lost there too.