My first open-source contribution was to FFmpeg, a toolkit for video and audio processing. I was trying to rip a DVD for my parents so that they could play it on their iPhone, and I was amazed by the lack of official documentation on such a simple task like re-encoding, and typos and mistakes that made reading docs even harder than it was. I thought it was within my ability to at least fix some of the obvious typos, and so I set out to do so.
With my fresh Ubuntu environment, I proceeded by cloning the tree and reading the page-long contribution guidelines. Even worse, English is not my first language, and a few years ago my English wasn't quite as good as it is right now. Reading this piece of document took me hours.
Alas, FFmpeg is one of those old-school C projects where everything is done through git format-patch, git send-email, and a Mailman mailing list (if you've never heard of these things, good). No GitHub or "pull requests" for me. It took me a long while to set everything up (from committer name to branches to email servers) and finally send out that patch to the mailing list. I had no previous experiences with Git, and I was so proud of achieving the feat of simply sending out the email.
That commit was speedily applied by one of the FFmpeg developers, and there you have it, my first ever OSS contribution. No, it wasn't a smooth experience. I didn't have the help of documentations in my native language, nor modern utilities such as GitHub pull request. But I did do something for other people, and the ability to do that was really empowering for me.