Welcome to Azeroth!
That just shows that your boomer brain is in good working order. 
NOBODY’s brain keeps up with an introduction to WoW. The sheer density of what there is to learn will cause a singularity in spacetime if you try to take it in too fast. 
The thing to do is know where to find information when you need it.
Wowhead wowhead.com is your source for quests, items.
Icy Veins https://www.icy-veins.com/wow/havoc-demon-hunter-pve-dps-easy-mode gives you advice on building and playing your class, although most of it won’t kick in until you hit level 70.
And if it’s in WoW, it’s on Youtube somewhere - though, be conscious that the game does change and add each expansion, and changes may have happened since older videos were made.
Newcomer Chat, in the game, may help.
The Wownoob Discord https://discord.gg/wownoob and Reddit https://www.reddit.com/r/wownoob/ are well worth joining. The Wownoob Discord may be a place you can hook up with other players.
If you have played an MMO before, any MMO, you are already light-years ahead of the average new player. You already know about maps and travel, and vendors, and quests, and repairing your gear when it takes damage.
It’s best to log out at an Inn, to store up rested experience, so you level faster for a while when you come back.
Basically, all you need to know while levelling is to look for the yellow exclamation marks, and do the quests. Keep following those and everything else will look after itself.
Your only purpose while levelling is to level, and enjoy yourself.
There is nothing you can miss, that you can’t come back to.
There is nothing you can do that will permanently damage your character.
And, of course, please post here anytime you have a question!