Hi! you have a few options I would not recommend deleting files but the best safe way is to use the Device maintenance app which will free up room if you still do not have enough room the options are using a SD card to expand storage and move things like photos/movies/(and some apps support being run for SD cards) or backing up photos/videos/music to a cloud type service, finally a OTG device can also be used as storage.
Hope thats some help 
