What are some interesting open source projects to contribute to? I wanna start to do some contributions to popular OS projects but I've never done that before so... Any recommendations?

Most of the large ones are already pretty well catered for, and to get into the game at this point would require a tremendous amount of time investment in understanding the codebase. So I'd say the likes of Git or the Linux kernel would be a poor place to start. You would do well to find a small fledgling project that you're passionate about and go from there.

You could take a look at openhatch.org , they teach you the basic skills to contribute to the open source.Might be a good place to start.https://quora.com/How-do-I-start-co... quora answer also might help(I haven't read it btw).

Start with software you use. Is any of it open source and written in a language that you want to write in? Check their GitHub issue trackers