There are a few things you can do:
- clear app caches
- remove apps you don't need
- delete app data for apps you don't use
- remove updates from apps you don't use
Once you've done this, install a file manager from the Play Store (I use a free one called File Manager) and use this to locate and delete (or move to SD if you have an SD card) files. I would start with videos (usually in DCIM/camera) as they tend to be the biggest files.
Alternatively, connect the phone to a PC using a USB cable then use Windows Explorer (Windows Key + E) to either move files to the PC or delete them.