Start small, I'd recommend packaging something for your distro and maintaining it for a little while to see how the responsibility feels. If you want a greater challenge, it's also good to practice closing Github issues on projects you're familiar with.
The wait can be 84-days to have your paperwork done to contribute to Emacs. A very ambitious option for a newbie is the Haiku port of Emacs. You can start by bug reporting and progress to bug squashing or documentation and design. Github is owned by Microsoft and isn't the only place to start.