S.P.
I am a professional photographer and you need to Format your card every time you use it after removing the images. For example, you put the card in your camera and before taking any pictures you MUST format it. Then shoot away. When you are done download all the pictures to your computer and erase them from the card. Then Format again and repeat the process. This will help insure the longevity of the card and the pictures you take.
Just a note, I buy all the memory cards off of Ebay. They are a fraction of the cost and you can usually get Free Shipping. Just be sure they are not shipping from another country. Only buy from the US or it will take forever. Good Luck!