I am taking this class next school year (junior year).
I heard it is very difficult. Is this true?
I’m a bit worried for it, and really don’t want to end up with a B in the class. What are some things that I can do this summer to better prepare myself for it?
Difficulty is highly dependent upon both the teacher and the student. Is it difficult compared to AP Physics C? No. Is it difficult compared to AP Physics 1 alone? Yes, but more due to the pace, IMO.
See if there are any online refresher courses for the class. Look at the AP website to get a feel for the curriculum.