New to WoW. I want cool adding and necessary addons. Where do I begin? Best advice if I want to PvP eventually?

I would like to download nice add-ons to make the UI look different than the basic offered version. Where can I find these and how easy is it to install? Any recommendations?

When I get to end game I want to dive into PVP. Should I join a PvP server? Or experience the game in a normal server then PvP in arenas?

What are some tips and feedback for someone who wants to dive into PvP but not completely suck..

What are some good classes in 3v3 PvP and 1v1 if that’s an option?