i have deleted a lot of stuff but it still shows 0 byte.. help!

Go to Settings > General > Storage & iCloud Usage > Manage Storage and clear up some space. There is section toward the top which will tell you how much space is used (Apps, Music, Videos, iOS, etc.) and how much is available. I'd recommend at least having over 200MB of available space.

Try to reboot your device by holding the Sleep/Wake button and the Home button until the screen turns black and the apple logo appears.

If the problem still persists, I would recommend setting up an appointment at your local Apple Store's Genius Bar. http://www.apple.com/retail/geniusbar/

Then restore the phone and setup as a new device. You probably have data corruption.

If you are still looking for a solution, please read this article first:

http://osxdaily.com/2013/07/24/remove-other-data-storage-iphone-ipad/

Then connect your phone to your computer, open iTunes and on the Summary Tab (the default) there should be a bar at the bottom showing your usage. Look at a category called "Other" and also any other categories to try to determine why you have zero available.

You will likely have to do a full restore to clear the issue per these instructions. https://support.apple.com/en-us/HT201252

You can try restoring from backup but if the problem comes back your system is corrupted and you would have to repeat the restore again but setting your device as new. You would have to reenter all you settings and apps from scratch, a time consuming process, but hopefully you will have a working iPhone again. Good luck.