what if I want to start my contribution I just don't know which project to choose ? this assumes that I already know the project I want to contribute to.
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.
For almost any project there is an infinite amount of work to do. Check out OpenHatch for opportunities, or just dive in for things you rely on. Big communities are great too, checkout Linux or one of the distros:
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.