Dear enthousiast,
Thanks for your interest. Before applying for GSoC with Sagemath,
you should start getting a little bit more familiar with Sagemath
(especially things related to symbolic expressions and the fast
callable). You should also try to compile Sage on your computer
and understand the development process which is organized around
the website
https://trac.sagemath.org/.
A good entry point is SageMath documentation
(
https://doc.sagemath.org/). Ideally, you could also start
contributing to Sagemath with beginner tickets.
Best wishes,
Vincent