Preparing for Math-CS?

I'll be entering UCSD in September. In high school, I found myself to be slightly above average in math and average in CS but I feel worried about what is to come after reading what people have said about the difficulty of classes. Can anyone recommend books/materials/lectures or anything I can study to prepare myself? I hope to use this summer wisely instead of playing CSGO and rewatching 9 seasons of The Office.

On an unrelated note, do a lot of people play CSGO in UCSD? If not, what videogames are popular? I am from the Philippines and don't know much about UCSD or San Diego in general.