Start with Mathematics for Machine Learning Specialization on Coursera. If starting from complete scratch, the topics you should certainly review/cover, in any order are as follows:
- Linear Algebra — Professor Strang’s textbook and MIT Open Courseware course are recommended for good reason. Khan Academy also has some great resources, and there is a helpful set of review notes from Stanford.
- Multivariate Calculus — Again, MIT Open Courseware has good courses, and so does Khan Academy.
- Probability — Stanford’s CS 229, a course I’ve mentioned later, has an awesome probability review worth checking out.