Thank you for your interest in Sagemath.
Before applying for GSoC with Sagemath, you should start getting familiar with Sagemath (how to install and use it, code some basic algorithms, etc.) and with the developer guide
http://doc.sagemath.org/html/en/developer/index.html (what’s the coding standard, how to use git and trac, etc.). Then, you can start contributing Sagemath with smalls patchs (implementation of a new algorithm or improvement of an existing one).