It is highly recommended that you become familiar with using Swift in your own projects before contributing directly to the language itself. We put together handy [Getting Started][get_started] guides with step-by-step instructions to get you up and running.
Many of the repositories in github.com/swiftlang have a CONTRIBUTING.md in the root of the repository or in the README. This is your pathway to submitting pull requests, getting code reviewed, and more, for that area of the project. If there is not a specific CONTRIBUTING.md listed, many projects will follow the standards set in the main Swift repository under /swift/CONTRIBUTING.md.