How to get started with a project as big as RoR?

I’m an intermediate RoR based web application developer, pursuing CSE from JIIT, India. I’ve some prior experience of collaborating with others on github but that has always been restricted to small scale projects. Contributing to projects like RoR quiet honestly, intimidates me. Please, tell me how to get started with a small contribution.

“Refactor Rails’ Cookie Implementation and Improve Signing with Expiry and Purpose” seems quiet enticing. If I get a chance, then I would like to go for it.

Thank you,

Amarjeet Singh Mudhar.