Mathematics in SR

Special relativity is remarkably simple, mathematically, as opposed to general relativity.

As with all physics, calculus would be helpful but most of special relativity requires nothing more than algebra.

 

An elementary course in matrices would help.

 

It depends how far you want to go I guess. To understand the fundamentals, you won't need much more than basic algebra, like the others said. If you go into more advanced formulations though, you would need to understand the concept of 4-vectors and tensors, which are considerably more difficult.