I can't get my memory cards to work on any of my cameras!?

I recently bought two digital cameras. My first camera, a Casio EXILIM only hold 5 pictures internally without a memory card so I bought one (Toshiba micro-SD brand), but when I insert the card the camera reads card error. Thinking there was a problem with the camera, I bought a Nikon Coolpix S203. This camera's internal memory is better than the Casio but when I inserted a brand new memory card (Lexar micro-SD brand) it also read that the card can't be used. I also tried the memory card I bought for my Casio but it won't work. I've tried unlocking the cards and formatting my memory on the Nikon. I don't understand why these cards aren't working. I have 2 different memory cards and 2 different cameras but they won't work in either one. What could be the problem?