Swift is a giant project and so much depends on what interests you, so you will have to explore for yourself then ask more specific questions about the areas that intrigue you. There is no shortage of things to do, so it all depends on what you feel comfortable handling and that sparks your interest.
As for subsystems that are more approachable, parts of the Swift toolchain like the package manager, compiler driver, and syntax parser are written in Swift, so newcomers would likely find those easier to contribute to. There are also a few parts of the compiler deeper than that being written in Swift, but that would require more compiler knowledge.
Use the tools, see what's missing in the GitHub issues, peruse the source in the repos that you find most relevant to your favorite topics, and then come back with questions specific to the areas you want to learn more about. 