Data Mining roots from Statistics and Machine learning. Here are some valuable free resources:

General Statistics

• Think Stats: Probability and Statistics for Programmers
• Differential Equations in Data Science Python Tutorial

Linear algebra

• Deep Learning Book, Chapter 2: Linear Algebra

Probability & statistics

• MIT 18.05, Introduction to Probability and Statistics
• All of Statistics: A Concise Course in Statistical Inference

Calculus

• Stanford CS231n: Derivatives, Backpropagation, and Vectorization

Machine Learning

• Andrew Ng’s Machine Learning
• An Introduction to Statistical Learning

Programming

• Learn Python the hard way

Resources:

• The Best Machine Learning Resources
• The Open Source Data Science Curriculum