Some basic tips on using the ActivityPub services (Matrix is its own beast and stands slightly apart from them but you can add your Matrix ID to the settings on Lemmy and it gives you a button for secure messaging)
Most will use markdown for formatting, as is used here.
In quite a few services you can use the @ address format to follow other accounts - on the microblogging services you can usually also use the same format to follow communities in the threadiverse (Lemmy/kbin). You can’t currently follow individuals on Lemmy (you can on kbin) but it is a much requested feature.
You can often reply on other services using an account on a different service.