Thanks for the question, I wrote about this in a post before: medium.com/@hzoo/ossthanks-some-thoughts-d0267706…
I was inspired by a bunch of projects like khan academy, d3, bootstrap, etc. But Jonathan Neal was the one got me to intentionally look into contributing into a project (github.com/angular/angular.js and then jscs.info).
BTW, You can use github.com/pulls to check someone's PRs
Technically my first PR was github.com/Khan/khan-exercises/pull/10765 (not merged since I didn't rebase correctly and didn't understand git at all).
My first PR to JSCS (the first project I eventually contributed regularly too and then became a maintainer for) was github.com/jscs-dev/node-jscs/pull/677 to update the table of contents.
My first PR to Babel was github.com/babel/babel/pull/932 to add ES6.
Thus all of my first PRs were very simple: typos, changing docs, basic refactoring/reformatting/updating tools.
This is probably part of the reason why I got interested in the tooling side of things and wanting to automate a lot of it (especially autofixing which lead me to work on JSCS)