To understand the theories, you first need to handle some basic algebra and linear algebra (working with matrices). Once you can handle that, it's a quick jump to tensors, which are a way of shortening a lot of the mathematics involved. Once one gets a handle of a couple of rules, working with tensors can be pretty striaghtforward. Tensor albegra provides the basics of the system in which one will then do calculus.

So you need:

algebra,

linear algbra,

tensor algebra,

calculus

You'll probably have to pick up a little geometry, but you are likely to learn enough learning the above areas.