Dear Aniket,
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 patches (implementation
of a new algorithm or improvement of an existing one).
You can see the current activity at
https://trac.sagemath.org/
Sincerely,
Vincent