Discrete mathematics is a very important subject and one of the foundations of computer science. If you want to start with discrete mathematics, I suggest you start with some basic concepts, such as set, function, relation, graph theory and so on. These concepts are the core of discrete mathematics. After mastering these contents, we can better understand other aspects of discrete mathematics.

Here are some resources I found about the introduction to discrete mathematics:

-Self-study guide to discrete mathematics

-discrete mathematics and its application

-Peking University-Discrete Mathematics (National Excellent Course)