Card is always trying to format. No matter what

I have a nikon d90 and when i put in my sd card. The camera is always saying that i have to format it. the worst thing is that it does this with every single card i put in. im fairly new to photography and the camera i have is from a friend he said he would lend me it for a couple months if i dont break it. But i geniuenly have no idra what to do now