Start anywhere. Steve Klabniak has a commit bit on Rails primarily because he showed up one day, said "The docs are a mess", and started writing better docs. As he wrote more he got better knowledge of the Rails internals. He then went through the issue tracker just for gardening, marking issues as duplicates, trying to repro them, etc, and suddenly he was the guy who both knew all the exposed features and all of the bugs. Then he put 2 and 2 together and said "Aha, I think I can probably independently fix this one. Here's a pull request with the patch and tests." And repeated that until they said "OK, OK, enough already, have a commit bit."

Can I make a suggestion though? What are your goals in life over the next few years? Does getting a commit bit on Rails represent the single best thing you could be doing to advance those goals? There are many, many OSS projects in the ecosystem which have lower barriers to entry and higher returns if you do them right. Some of them don't exist yet, but you could solo-deliver them in a number of weeks. There are many plausible goals you could have where solo-delivered gem X is vastly more valuable to you than "Got 3 patches accepted into Rails core."