Default [Q] how to backup stock ROM in NON ROOTED MTK

I new here. and I hope that Developers will help of New comer.
I have china's MTK 6517 phone.
NON ROOTED
Android version 4.1.1
Kernel version 3.4.0
so I want to install custom ROM.
Before installing custom ROM I want to Backup my Stock ROM.
coz if made mistake I will reinstall my stock ROM.
AND WHAT IS Difference between Stock ROM and Official ROM.
AND which ROM i can install in it?
sorry for bad English.