1. As long as the card is Class 10 or U1, it's plenty fast enough. (No one sells Class 6 or slower cards any more.)
2. Format it as external storage.
3. Use a file manager to copy files to the card. Once a copy is successful, delete the one in internal storage. (Don't "move" a file - if the move blows in the middle, you can be left with no file. If a copy blows, the original is still there.)