I’ve been waiting for time to do the same, but haven’t yet contributed anything. From what I can tell, you want to look here for guidance: Swift.org - Contributing
The “good first issues” may be what you’re thinking of, but there are lots of ways to contribute.
Important to note, you don't have to necessarily contribute only to the Swift compiler in the main apple/swift repository. There are plenty of tools included with the Swift toolchain, e.g. SwiftPM, Swift Driver, SourceKit-LSP, Swift SDK Generator, and of course libraries, such as Swift Collections, Swift Async Algorithms and so on. You can find plenty of different areas to contribute to that can fit better your set of interests.
There are a large number of GitHub issues tagged “good first issue”. You might consider starting with one of those: Issues · apple/swift · GitHub