I found this question on Quora.
“So I’m looking to study Computer Science and I am retaking high school ( I am 20 ).I can only take 3 subjects, I have chosen English, Pure MATH, and the third one I have no idea what to study. I get to choose out of HS subjects ( physics, Chem, bio, history etc ) please help”
It’s unfortunate that you don’t have computer science or a computer programming language as an option.
If you have the option, look for something that teaches critical thinking skills, such as philosophy. You’re already taking a math class. You would benefit from more classes that exercise your ability to solve problems.