What version of Android is it running?
That's not a lot of storage space, honestly, so I'm not surprised it's filling up. You'll probably need to clear cache regularly. Open Settings, then go to Storage. Tap "Cached data," which should prompt you with something like, "This will clear cached data for all apps." Select OK.
I'd also look at media storage and move as much as possible (all photos, videos and music files) to the memory card.