Dear Bahar,
Thank you for your interest. I would suggest getting familiar with
the developer guide
https://doc.sagemath.org/html/en/developer/index.html
(what’s the coding standard, how to use git and trac, etc.).
Furthermore, I think it would be good to make a small contribution
to SageMath in order to learn about our development process. You can
see open tickets on our trac server:
https://trac.sagemath.org
Best,
Vincent