ROOT is basically a library of (a lot of) classes and things for C++, so in my opinion you at least need to learn how to express yourself in C++, and how an object oriented language like C++ works. To understand the things that ROOT can do it is good if you know stuff about how data is analysed in particle physics, what different plots are used for, different types of variables, special fitting functions (Breit-Wigner for example), some statistics etc.I know it's not very easy to learn ROOT though, I've tried myself and if you're an absolute beginner it's very tough.
Personally, I would begin by learning the basics of C++, the syntax, different basic classes etc. Then I would start focusing on the physics stuff.