You didn't say if the card just won't mount, or you are only having trouble moving the files. If it won't mount on the device, then the card may be the wrong kind for it (yes that can happen) or it could have to much storage to work with your device (yes devices have limits on this).
If you want to just move files (photos, videos, music, etc.), not apps, to the SD card you can use the file manager app that is on the device, or if there isn't one you can download and install one to access and move the files. Sadly, they usually only let you move one or two at a time.
To move them all at once, or in large amounts, you can connect the phone to a computer via USB cord, then move the files over to an empty folder on the computer. Once...