Hey PK! Welcome to the JavaScript community!
I think that you've got the right idea to start your JavaScript journey getting into open source! There are so many things you can learn from open source.
As for getting better at it. I think the best thing you can do is just do it a lot! There's no shortcut to experience. It just takes a lot of time. I did answer a similar question on my GitHub AMA which you might find interesting (also this one).
Also, you might be interested in this blog post as well.
Good luck!