If most of your message conversations are useless, you can long-press on any conversation, then check boxes should appear, and you can tick all the conersations you don't need and tap the Delete icon.
Again, you're suggested to analyze your phone memory first. If you do that, you may find text messages only take up a small amount of memory. As I said, go to Settings> Storage, tap Used space, and then check every item listed. On my Samsung, there are Applications, Pictures, videos, Audio and Downloads.
On Applications, the apps should be listed according to size, some of which can be 300 MB or over (games can be much bigger). Here you can uninstall, clear cache and data (not suggested if you still need the app), or move some apps to SD card.
For pictures, videos, audio and downloads, you can either view them with the default apps or via My Files app (assuming your s7 has an app on the same name). If you use My Files app, most images are on DCIM folder.
Important: Back up your data and files to PC or cloud first.