Sorry, bit of a long post here 
No macro needed for this.
Basically, press the ability when you have an enemy targeted, and if your pet is within range(50 yards req for Kill Command, 40 yards req for Barbed Shot), then your pet will charge them and perform the attack.
Kill Command cannot be used unless your pet is within 50 yards of the target enemy.
Barbed Shot has a double range indication/condition. It is a ranged attack based on your own position, so even if your pet is not in range of the target, you can still fire the ability if you are within range of it.
For your pet to get the charge, it needs to be within 40 yards of the target.
As for macros, you’d do well to attach a “cast pet-basic attack” command to each of your offensive abilities.
Like this:
#showtooltip
/cast Barbed Shot
/cast Bite
#showtooltip
/cast Cobra Shot
/cast Bite
(or Claw or Smack, depending on which type of pet you have and what Basic Attack it comes with, check your spellbook(pet tab) to find out which it is)
And so on…
Why would you want this type of macro?
Because the Pet UI is quite horrible when it comes to the execution of some auto-cast abilities for w/e reason.
If you look at your pet’s Basic Attack(Bite/Claw/Smack) when it’s on auto-cast, while your pet is attacking an enemy, you’ll notice how it can take between 0.5-1sec after the ability comes off CD before the pet actually casts it again.
Meaning: Macroing basic attacks into your own abilities/actions will, as long as you continue to press buttons, guarantee that your pet uses Basic Attacks as soon as they are ready.
The Macro’s below aren’t for DPS but as a BM hunter especially, they are incredibly useful for just general gameplay. As they take away the need to switch targets during combat to use certain abilities.
Misdirection
/showtooltip Misdirection
/cast [target=mouseover,help,exists,nodead][@focus,help][@pet,exists] Misdirection
This one has 3 conditions
Prio 1: Cast Misdirection on any friendly unit(frame or ingame character) that you have your cursor placed over.
Prio 2: If no friendly mouseover unit is available, Misdirection will be automatically cast on any friendly unit you have set as a Focus-target(Focus frame).
Prio 3: If none of the above are met, then Misdirection will be automatically cast on your pet if alive and within range.
The next macro is exclusive to BM hunters as, they are the only ones who can tame Spirit Beasts as pets.
Spirit Mend
#showtooltip Spirit Mend
/cast [target=mouseover,help,exists,nodead][@player]Spirit Mend
It has 2 conditions
Prio 1: Spirit Mend will be cast on any friendly unit you have your cursor placed over.
Prio 2: If no friendly mouseover unit is available, then Spirit Mend will be cast on you(the player).
You can change this one to “@pet” or basically any specific unit you want that is friendly to you.
The Spirit Mend-ability itself is a fairly strong instant heal + heal over time effect on a fairly short CD.
So you’d do well to make use of it in case you run with a Spirit Beast as a pet.
By default, you can see it on your Pet Unitframe. It’s called “Frenzy”.
If you have any addon that alters your UI(unit frames) somehow, I’d recommend you to check where your unit-specific buffs are displayed. As, some mods change things like Buff-placement.
Specifically, where your pet’s buffs are displayed.
You’ll learn these things eventually. 
Just keep looking at everything and read what individual abilities and effects do.
Then try to manually test them out if you’re not sure how they work.
People learn in different ways.
Both.
Azerite armor you equip, as you can see, comes with 2 outer rings that contains various damaging traits that are spec-specific, for the most part.
Try to find azerite armor which has Primal Instincts on one ring and Dance of Death on the other.
For more details/specifics, check Icy Veins.
Essences are a end-game progression system for our characters to up our power levels.
Essentially.
You can get Essences from various activities, such as Island Expeditions, PvP, Raids etc.
Each essence come with multiple Ranks for you to upgrade your essence with.
Essences are permanent(for now) and are character-exclusive(not account-wide).
https://www.wowhead.com/guides/heart-of-azeroth-essence-overview
Here you got the full rundown on what Essences are and how to obtain them.