I am sure Steve would say this as well (because he has said it many times). Start by improving the documentation of helping to regress bugs. These two items while seemingly inconsequential are actually a huge help to the core team and all developers who actually use rails (especially awesome documentation).
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.