Usually people begin by contributing to projects which they already use. For a beginner, it might be better to start contributing to a project which is mainly written in the language which they're familiar with. Also, try to find a project which is beginner friendly; they usually have some issues marked as 'beginner' or 'good first bug', and also have a CONTRIBUTING.md page.

Bonus: A good idea is to start contributing to a project which contain problems sets to be solved by coding the solutions in a particular language. You could send PRs for creating/implementing some problems. http://exercism.io/ is a good example.