There are lots of guides on the internet that tackle this issue. But be aware that some people refer to the phone's built in storage as "internal storage", but that's not what Android means. There is (at least on my phone which is on 4.2.2 still) a "root" partition that is only 1GB big. This is referred to as internal storage. The rest is called "Phone storage" in Android.
You can have a look at what's eating your internal storage using the tool DiskUsage from the play store. Usually it's all the apps. The properly developed ones will give the the option to move data files from the internal storage to the phone storage. This can be done from the app details page:
What I usually do is go to
Settings->Storage. In the "Internal Storage" section, wait until it calculated the sizes, then click on the
apps (app data & media content) bar. You'll be taken to a list of apps sorted by size. Now click on each entry starting from the top and select
Move to internal storage wherever possible.
This should help quite a bit if you haven't done it before.
To make your phone do the right thing for new apps, go back to
Settings->Storage and tap
Preferred install location. Set it to