Start by taking several introductory courses in computer science. In my college, computer science majors almost always take the following two classes in their freshman year:
- CS 1: Introduction to Computer Programming – In this class, you will learn the fundamentals of programming through the programming language Python
- CS 2: Introduction to Programming Methods – In this class, you will learn about data structures, implementation and analysis of common algorithms, and other various programming techniques
Furthermore, you should take a Discrete Mathematics course to learn about graph theory, algorithms, networks, etc. It will help you develop a strong foundation for algorithm/theory-intensive computer science courses.