Well, depending on which one you got, that phone should have either 8GB or 16GB of "Internal" storage. Also, it does allow for a SD card. Max size 256GB.
The phone's storage, however, is taken up mostly by the OS and pre-installed apps. Any apps you add to it will take up more space. Plus, when you use apps like browser, Facebook, Messenger, etc., they put lots of data on the device. So if you don't clear out the cache, history, old texts, etc., they start to take up a lot of space as well.
First I would do that, clean up the apps you do use. Just be sure to save any passwords, important info, etc., before cleaning them up. Then go to the apps on the phone, that came with the phone, and see if ones you never use (and are not important...