I wrote up my own personal 'contributing to Rails' guide here: http://blog.steveklabnik.com/posts/2012-07-05-how-can-i-cont...
The official one is here: http://edgeguides.rubyonrails.org/contributing_to_ruby_on_ra...
The easiest way is to write documentation; that's how I got started. It's not for everyone, though. The simplest possible way to get involved with writing code is to pick an open bug on the tracker and fix it.
Like most things, it just gets easier the more you do it. The first one seems hard, everything seems confusing. But by the time you've done two or three, you wonder why you ever thought it was difficult.
Once you get a certain level of familiarity with the way Rails is built, find a small bug and follow the contribution guidelines.
There is no magic :) Take small steps and continue to make any contribution you can. Ask a ton fo questions and work through issues, you'll get there.
Here's a blog post that Steve actually wrote about contributions:
http://blog.steveklabnik.com/posts/2012-07-05-how-can-i-cont...
Congrats as well Steve! You should be really proud of this release with the rest of the core team.