If you're looking for a way to contribute back to Rails, the quickest way to get your first patch in is by improving its documentation.
Some people don't find patching documentation as sexy as slinging code, but I've come to love the humble doc patch. Here's why they're so great:
It's easy to get started. Your first patch can be as simple as fixing a typo in a Rails Guide. Later, when you're feeling more at ease with the process, you can improve the documentation for a class or method right in the code. Find an undocumented class, dig into its guts, and write a good summary. Ever read the examples for a method and think they could be improved? Don't grimace and move on; fix it!
Your patch will be merged in quickly. All patches to Rails' documentation go into the docrails repository. This repo has an OPEN COMMIT POLICY: you simply message lifo on github and he'll give you commit rights. You can make your patches at will, and the repo is regularly merged with the official Rails repository (here's the most recent example of that happening). There's no lengthy signoff process, so your commits will never languish while waiting for "+1s" or attention from a core member.
Most people don't want to do it. It's actually pretty tough to find bugs in Rails to fix. There are so many Rubyists crawling all over it that there's just not that much low-hanging fruit to patch. However, most developers don't really like writing prose, so there's a lot of easily-improved or outright missing documentation out there.
Your commits count toward the Rails contributors leader board. If you're into that sort of ladder-climbing, you can break into a top 200 spot with just 8 commits. With just a bit of effort I reached 267th, which gets me laid all the time.
It feels damn good to give back to projects you use every day, and seeing your name in the commit logs is a real trip at first. If you've been looking to start making some open-source contributions, you could do a lot worse than improving documentation used by thousands.
If I've inspired you, you should start here: the Contributing to Rails Guide has a section specifically for working on documentation. Good luck!