As a new lemmy user I've got some question?

1. When I use the https://join-lemmy.org/instances do I need to register a new account for example beehaw.org, sopuli.xyz and other server?
2. From this video https://youtu.be/UirZ9zPCGtQ?t=155 he said that I can see other communities from "all" so that includes like beehaw.org and more?
3. What are some tips that I should know to enjoy lemmy?

edit* how do you turn off the lemmy notifications. It covers my screen.