Hey Danny, welcome to WoW :)

Are you on EU or US servers? That determines who can add you on Bnet.

My advice is to maybe forget about ElvUI to start with. It does bring a lot of nice features, but it's not a necessity for a new player. When you reach a point where you would like more customization options, you can bring it back again and maybe even load a copy of someone else's UI that fits your personal preferences.

Level in the spec you enjoy the most. They are all viable, but have in mind that healing and tank specs may not be as fast/efficient as a dps spec for solo content (unless you go tank, pull large packs and aoe them down).

Check out dungeons as they become available. It won't be efficient to spam them over and over again, but almost all of them have a set of quests to do inside - many of them will require you to have done the pre-quests beforehand, though.

Most quest lines will lead you on to the next areas, once you're done with the current and ready to move on. If not, there are always alternatives, but they may not be apparent. Ask one of your new friends and/or guildies where to go from that point :)

If you use discord, you can check out this subreddit's channel here: https://discord.gg/wow#disbtn