You would have to remove/uninstall apps you can (many preinstalled ones won't uninstall) or delete files you have on the device (photos, music, etc.) or move files from the device to either a SD card, computer, cloud account or other location off the phone.
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 there, and still connected via...