That's depends on what you want to learn and for why, well, some people want to understand the formalism of a theory, as theoretical computer science or theoretical physics where others are only interested in applications, so I will take a generalist approach in the topics, yes topics not books, that I will advise you to learn. Unless the book is awful (and there are many out there that are) it will makes no difference which book you pick, you generally will not "read" a math book, the only case in which you will is when it's a book for divulgation (as Polya's "How To Solve It"). For me the basics is: Statistics (Descriptive and some Probability), Calculus, the idea of Limits, Derivatives and Integrals (for Multiple Variables) and applications, Linear Algebra and also some Applications (there are many), numerical Linear Algebra is totally necessary if you want to apply it in the real world, a basics in Differential Equations, some Numerical Analysis.

If you want to learn things closer to computer science then learn something of Number Theory, some Enumerative Combinatorics and Graph Theory as well. The list is extensive because I come from a mathematical background. If you learn at least a bit of these topics them the next step will be apparent for you.