How to Get Started in Computer Science
Theoretical computer science means different things to different people. However, here are some areas which are commonly taught in undergraduate theoretical computer science classes:


Combinatorics and elementary set theory. This serves as prerequisite to all that follows.
Data structures and algorithms. One standard textbook is Introduction to algorithms, but it might be too advanced if you're just starting out.
Computability and automata theory. One standard textbook is Introduction to the theory of computation by Michael Sipser.


In some places, another core subject is functional programming and labmda calculus.

An undergraduate degree in computer science typically deals with several other topics, such as operating systems, networking, and sometimes compilation.