Use My Files to move them. Use copy and paste as sometimes the move command can go wrong leading to loss of files. Once you are happy all files moved successfully you can free up the space in internal storage.
thats the method I use what Pugs1957 has described but another way is to go into gallery... hold and mark all files that you want to move to one particular album... in the options it asks you to create a album... and then asks you to have the new album on phone or SD card.
you can also go into camera and camera settings to update where future pics and videos store in too.
