I think some of the previous responses may have missed the part where the OP said he/she was a high school student. There are many different mathematical levels at which one can study GR. IMO what is most difficult about GR is not the math. GR requires a highly developed level of *physical* understanding.
In direct reply to the subject line of the OP ("list of required branches of mathematics to study GR"), my list is:
-high school geometry
-high school algebra
I would suggest starting with nonmathematical treatments of relativity. One good one is the one in Hewitt, Conceptual Physics. There is very little math in that book, but anyone who really deeply understands the relativity presented in it knows a lot of relativity. For example, it has a very good treatment of a twin paradox, which gets posted about endlessly here on PF. Another good nonmathematical book is Gardner, Relativity Simply Explained. (It's a little out of date, though.) Also: The First Three Minutes.
With nothing more than calculus, one can do the treatment of GR in the Feynman Lectures, and also Exploring Black Holes by Taylor and Wheeler, and Spacetime Physics.