"Storage space is running" or "storage space is running low"? If it's running low then, as chanchan said, delete some things you don't need, uninstall apps you installed but aren't using any more, move things you don't need on the phone to the cloud.
There's only 8GB of storage space, a lot of it taken by the phone's firmware, and most of it taken by files you can't see because they're hidden (system files). An 8GB phone is large enough for Phone, Messages, Gmail and not much more, which is why phones are now coming out with 128GB of storage.