Because you'd be joining a project with other devs working it, you'll be able to start with small changes and progress to bigger ones. As you're not the only dev, you don't need to get the whole thing done, just your little piece.
As the project already exists, you'll be able to look at the code's design and work with the other devs to learn.
Like you, when I wanted to learn Ruby, I was nervous biting off a big project. So I worked on the org-ruby gem. I only made a tiny mod to it but that gem is one of the gems that Github uses. IMO, that's very cool.
You can look at the pull request history for the work I did at https://github.com/bdewey/org-ruby/pull/20 - If you read the interchange I had with the developer, you'll see that he really helped me understand Ruby, Rakefiles, and the org-ruby gem.
I imagine that the gem owner spent more time helping me than it would have taken him to implement what I did, but he seemed happy to help.
When you think you've found a good project, you may want to send the devs the question you posted here and see what they say. Perhaps they'll be thrilled about the help, perhaps not. Better to figure it out before you start.