Hmm, it's a bit of a toughy to say as there are lots of things that can eat up storage!
I'd start by going to Settings>General>Storage & iCloud Usage>Manage Storage. That should give you an immediate idea of what's really taking up the space. If you have a lot of storage used from Messages, you can just simply delete conversations. Quite often that really does help.
How's your music collection? If you use Spotify/Apple Music, it'd free up some space to stream your music rather than keep them stored locally. Have a sift through the photos too, see if there's anything that you can delete.
You can now remove stock apps if that helps. But beyond removing apps you don't need, there's not much you can do.
Please feel free to quote/reply to this if you have any queries. If you could post a screenshot of the storage usage, that'd be great too!
Best wishes.