Howdy! I am also starting to dip my toes into Lemmy/fediverse dev (I'm toying with making an iOS client). My background is more webdev (specifically NodeJS and React).
I think dipping into the README's for the project would be useful. Really depends on what you want to hack on.
If you're more interested in the backend (Rust) side of things, you can check out the repo here: https://github.com/LemmyNet/lemmy
If you'd like to get your feet wet with the frontend (like you said it's mainly a TypeScript/ReactJS app), that's over here: https://github.com/LemmyNet/lemmy-ui
You can run both the frontend and backend locally via a docker container, and test your changes there.
Would love to find a good place to chat about Lemmy dev - having never worked on a decentralized app I have a ton of questions!