Depending on what kind of person you are, a different strategy would be in order, i.e if you tend to like being thrown in the deep-end try tackling the biggest github open issue you think you can handle.
If your the type to ease into things, find good-first-issues of projects you like: <self-plug> checkout, feel free to reach out! :) https://github.com/nabaz-io/nabaz/issues/2
Find a small project that you're interested in and check out the developmental discussion channels to see if there's any way you can lend a hand. Starting with trying to find complex coding tasks in large bureaucratic projects is setting yourself up for failure IMO.
For example, https://zynaddsubfx.sourceforge.io/contribute.html shows a few different levels of involvement in one project I maintain. That's not me saying you should work on this project specifically though; you'll want to find projects that you're enthusiastic to learn about.