That was a barely usable phone 2-1/2 years ago - with only 8GB total storage. You have to be ultra-careful to close every app when you're finished with it, to allow it to clear its cache. (If you use the Home button when you finish using an app, you're assuring that you'll run out of storage.) And now that you've had over 2 years to get used to Android, and decide that you like it, you should get a real phone - at least 64GB of storage. (They make starter phones that will barely run what comes on them, but you don't use them for months, you use them to get used to Android, to see if you like it - then if you do, you buy a real phone.)
If you want to see what you have, install a terminal app, or connect the phone to a PC and run adb and issue the shell command. Then issue the
df
command, and you'll see how much space there is on the emulated or sdcard (that's not the SD card - that's extSdCard) partition - probably not more than 2GB. The rest is used by the phone.