You'll want to learn Git if you haven't yet. It's by far the most common tool to collaborate on code in open source (and outside, to a lesser extent).
Usually I find projects because I use them but they don't quite do what I want, or there's a big. But Github also has a decent search function and list popular repos for the week, to find interesting stuff.
Some projects have issues that are specifically labelled as "good first issue" or something like that. That'd be a good place to start.