MiFare uses encryption in the communication between the PICC (card) and the PCD (reader). First, you'll need a reader that supports MiFare. The PN532 based shields do support Mifare, so you're good on that part. The MiFare card has a special memory format, it's not just a sequence of bytes read/written sequentially. There are two key things you must do: 1, understand the memory format and 2, correctly authenticate. The MiFare card has keys written into it, there is a standard key when you buy new cards, you can change the key, but you must remember it or you won't be able to read/write from the tag later. When you want to communicate with the tag, you must first authenticate with the keys, then you may read/write.