I have been having trouble with my card reader and used it yesterday to download some shots from my 16Gb card. All downloaded Ok but when I put it in the back this morning it told me there was a problem and to reformat it in my computer. Do I need a special program to do this?

I have an iMac 2.8 GHz INtel Core 2 Duo running 10.5.8. I have thrown out the card reader and ordered a new Lexar one online, hopefully it will be here at the end of the week. Obviously I won't be able to reformat until the new reader gets here but I want to be prepared.