Computer Sci is just embedded sys, learning about the cpu and parts of pcs how they work, binary, denary, bits bytes and the rest of it, ethical and unethical uses of computers, lans wans and mans, some other shiz I forgot now. But you can google most of what I said and learn what I had to for my GCSE in it
GitLab > GitHub. Much better user interface, project timelining, and you can self host repositories for free, away from the web if you wish.
There are full computer science lectures on youtube from universities like Stanford which you can use to understand the theory.. additionally you can find university problem sheets online with solutions to see if you understand everything. Then you can find tutorials on building real life projects no matter how simple so you can get a basic understanding. Then GitHub is useful for looking at the source cods for anything you're looking to build which you can then edit to whatever you want. Best of luck!