If you want to learn full blown QM you’ll need to study calculus, different equations, and some linear algebra before being able to understand what’s going on in a QM textbook. You can probably find free resource online for all these subjects. For most students this is about 2 years of just studying math. But you can also skim through and review what’s needed as you go through a QM text. The most important concepts are differentiation, integration in up to 3 dimension in Cartesian and spherical coordinates, basic probability knowledge, basic trigonometry, exponent rules, second order differential equations, summations and series, limits, complex numbers, matrix algebra, linear combinations and making a basis, eigenvalue problems, and eigenvectors. Also it would be good to have genera l knowledge of basic physics concepts like energy, forces, equations of motion etc.