To add to the answer already given, keep in mind that it depends on what you want to learn.

Theory from the point of view of physics (as in, less focus on algorithms, but some focus on building hardware): see Preskill's notes or Chuang&Nielsen's book.

I would suggest quickly skimming them once before trying to read them cover to cover.

And some lighter online introductions might help as a first step.

You definitely will need very good knowledge of college level linear algebra (linear operators, basis, diagonalization, eigenvalues).