It really depends on what you want to research within string theory, but it's one of most mathematically intensive areas within physics. List a mathematical discipline, and chances are you can apply it within string theory.
At a bare minimum, you'll need everything through quantum field theory and general relativity, which includes calculus of variations, complex analysis, group theory, PDEs, path integrals, differential geometry, maybe some topology and anything else I've forgotten. String theory usually builds on this with at the very least a little algebraic geometry. If you take cues from people on the nLab, category theory can be big in string theory. Pick any combination of differential/algebraic and geometry/topology and it will be useful in string theory. Number theory as well.
Pick some math and learn it and it's hard to go wrong.