Hi Dhia,
If you have any questions about the project we can discuss them here on the mailing list or you can contact the mentors. Once you feel ready, please fill the application on the GSoC website: https://summerofcode.withgoogle.com/student-signup/.
You can find all the instructions (including a link to the application template) here: http://weblog.rubyonrails.org/2017/3/2/google-summer-of-code-2017/.