Hello! I wanted to follow up Alex’s post with another one where we consolidate information about Hacktoberfest for new contributors. You can sign up for Hacktoberfest here
and you’ll get a free t-shirt if you submit 4 pull requests to any open source project during the month of October. In any case, signing up earns you stickers. 
Once you’ve signed up, check out CONTRIBUTING.md for instructions on where to start learning Julia, how to get Julia set up on your system, and how to make a contribution.
For ideas on what to contribute, you can check out our issue tracker, paying special attention to anything with a hacktoberfest or #Intro issue label:
Of course, there are all sorts of things you can do to help out that won’t be listed on the issue tracker. For example, you might find a typo in the documentation or notice a docstring that lacks examples or isn’t adequately clear. Furthermore, you can make a difference without contributing to the base language! For example, if you have ideas for how to improve a tutorial like the ones below, those would be super helpful (and count towards your free t-shirt! :D).
If you’re unsure about how to get started, please ask questions!