What Mathematics Do You Actually Need?

Essential, sooner rather than later: complex numbers, vectors and matrices, inner products, and basic probability. This is genuinely most of what you need to understand a single qubit and simple circuits. Useful once you go further: Dirac (bra-ket) notation, which is mostly a compact way of writing what you already know, tensor products (for reasoning about multiple qubits together), and eigenvalues/eigenvectors (for understanding measurement more deeply).

The common mistake is treating this list as a wall to clear before starting. It doesn't have to be sequential: the math is more intuitive when it's learned alongside the physics it's describing, not as an abstract prerequisite months beforehand.