You might be surprised how many random videos you have stored on your iPhone, some you probably don’t even remember taking. You’ll find these, along with screenshots you may have snapped and no longer need, right in the “Albums” list inside Photos.
Go through and delete all the junk. While you’re there, pop into the “Bursts” album and go through each one. Bursts can snap hundreds of photos in just seconds, and you might not know if you’ve done it.
Take this as a bit of a lesson, though: if you can swing it, always try to buy one model up from the base storage offered by Apple.