There is no need to login to other instances. Just browse them, and if you want to comment something copy the link and paste it in threads search (Lemmy) or microblog (mastadon) and you can interact with them. I hope in the future apps will make browsing much easier and seamless without copying and pasting links around.
There are browser add-ons for the desktop that help with that process.
I recommend The Rust book too. Great for beginners. If you are more advanced developer the Tokio runtime documentation has a tutorial on how to create a mini redis clone https://tokio.rs/tokio/tutorial