Welcome back!
It’s been a long time. Things have changed. Give yourself a break - this is like learning all over again, at least for the first week or so.
Here’s a guide to what has changed since Mists, but I don’t especially recommend studying it too hard. https://www.wowhead.com/returning-player-guide-changes-mists-of-pandaria
If things don’t seem to be as you remember them, they probably aren’t.
Or it could be your memory. I notice a lot of returners remembering things that were never in the game at all. It happens. 
I agree with Lynlia that there is no point in trying to push hard for progress in Shadowlands. Dragonflight arrives November 28, and we are all expecting pre-patch around Oct 25.
I would recommend getting your character or characters levelled to 50, ready for pre-patch.
I am assuming that a) you haven’t bought Shadowlands and b) that Shadowlands will become free one pre-patch hits. So I think you can get your characters to 50, which is the pre-Shadowlands level, and then go the rest of the way to 60 during pre-patch, without habing to pay for Shadowlands.
Levelling is MUCH MUCH faster than you remember.
There is now an Embassy in each of Stormwind and Orgrimmar. You will find it on the map. In Stormwind it is beside the Lake. Chromie is outside it under a tree. If you speak to her, she will synchronise you with any expansion, so you can level to 50 in any expansion you like. We call this Chromie Time.
You haven’t seen Warlords of Draenor, Legion, or Battle for Azeroth. You can pick any one of the three to get you to 50. Personally, I’d go with either of the first two. Warlords has the famous Garrison, and Legion has the Legendary Weapons, and both have excellent questing and zones.
For reference, the sites youwill want to bookmark are:
https://www.icy-veins.com/wow/ for all about building and playing your class
https://www.wowhead.com/ for everything else: quests, items, maps, all that.
YouTube has everything you might want to know, as well!
This is an absolutely MASSIVE 2-hour+ video, but broken up into 30-60 second chapters, and I recommend it as a quick refresher of anything you’re uncertain about.
The only thing it doesn’t cover is Chromie Time.
If you have any more questions, PLEASE post again!