Well you can't delete all the internal storage. That is where your OS (operating system), preinstalled non-removable apps, etc. are.
You can move your own photos, videos, music, etc. to either an SD card, cloud account or a computer. You can also uninstall all the apps you don't really use (except preinstalled ones).
A full fresh clear out, would be a "Hard Reset" which would set it back to what it was when you got it.