Sadly quite a lot of people have issues with this. As for going about fixing it, try installing Samsung Kies on a computer and use that to back up some stuff. Then you might also want to just copy files directly from the device with Windows Explorer. Then you should be able to boot it into recovery mode (Power + Home + Volume up) and using the volume up/down buttons, move the selection over to "Format data" or similar and use the power button to select it. After the format, set up your device then you should be able to
A) Restore backup and
B) Retransfer (via USB connection) every thing not backed up.
Once your device is set up I would highly recommend you:
A) Download a antivirus - They may seem useless on mobile but they do help
B) Install some kind of no-root backup app - Simply backing up stuff to Drive/Dropbox will keep pictures and files and something like SMS Backup + will help with stuff like call logs/texts
C) If possible, look into rooting for much more complete backups such as Titanium Backup
