What should I do next to prepare for learning C?

If Beej’s book is making sense overall, you don’t need to abandon it just because someone online dislikes it. Work through the examples by typing them yourself, compiling them, changing things, and fixing the errors. You won’t understand every detail on the first pass, and that’s completely normal. Any preparation you do now will make the college course easier, even if the class starts with the basics.

Try a small text-based tic-tac-toe game. It uses ordinary C rather than requiring a graphics library: represent the board, read and validate moves, update the board, print it, and check for wins or draws. Once that works, you could add a computer opponent. It’s a manageable project that gives you practice with arrays, functions, loops, conditionals, and input handling while also showing you what topics you need to look up.

Before choosing a project, identify the specific parts of C that are confusing you. Then make tiny programs focused on one idea at a time—for example, a program using arrays, one that reads a file, or one that allocates memory dynamically. You don’t need to know the entire language or prove that you can build your dream project before class starts. Learning by making small programs and consulting the course material when you get stuck is a better path than trying to master everything in advance.

Find out which textbook, compiler, and development setup your college course expects, and practice with those if possible. That will be more useful than switching resources based on random opinions. A graphics library can be fun later, but it also introduces another layer of APIs and setup; for now, a console project will let you concentrate on learning C itself.