If you don't know what Open Source project to contribute to you can try to use the Explore button on GitHub.
It leads you to a page where you can filter the projects by topics, programming languages, or you can look at various curated collections.

Topics

For each GitHub repository the owner can add any number of "topics". As the user edits the list of topics GitHub offers some words based on what the user started to type, but at the end any owner can add any topic. So for example there are projects tagged as websites others tagged as website or web.

There are also projects tagged with esperanto and ladino (these are mine).

There are tags such as travel, newsletter, or icecream.

Languages

Once you selected a topic you can further filter the projects by language. This means programming language as recognized by the linguist tool of GitHub. See what they say about repository languages

Topics + Languages

If you are into Perl development you could select the Perl topic that lists 4,199 public repositories then you could further filter to Perl language in the Perl topic that has 3,063 repositories. Then you could sort the project by popularity or last update time.

Collections

Collections are community curated lists of projects. For example government apps or Made in Africa or open journalism.

How do you pick projects to contribute to?

I would love to hear what is your method of picking a project to contribute to. Any of the above? Something else?