Well the phone may have started out with very little space. Since you don't mention the phone I can't say how much, but you can do a google search for the specs (specifications) for that device and see how much space it comes with. Then take note that much of that will be taken up by the OS and the preinstalled apps.
Now things that can take up more space are apps like Facebook, a Browser, etc. They leave all kinds of stuff on the device when you use them. This is will leave you with less and less.
Go into your "Settings" and check how much space is being used by what. Then you may want to look at what apps you can uninstall (if any) that you never use and are not required. Then consider clearing the history and cache of things like...