I think most contributions to open source projects come from a necessity, so you can start to create web apps, a todo list, a chat, whatever you want, and eventually when you start to use open source projects (usually gems if you use RoR) to build something you'll probably see some features that would be cool to have.

Even some whole open source projects are created because someone needed something and after googled it didn't find it.

If you can make money and/or help someone while you are building those real projects would be awesome, if not, create new things from scratch, and eventually you'll create your open source projects or at least you'll make pull requests because you needed something for that project.

I hope this helps, thanks for sharing your story :)