If you install a file manager from Play (I use a free one called File Manager) you could use that to move files from phone storage to the SD card.
Alternatively, you could connect the phone to your PC via a USB and use Windows Explorer (Window Key + E) to move the files around.
I would imagine photographs and MP3s are the place to start, but if you look in Settings at Storage, you should be able to see where most of your space has gone.