I would say there are two, maybe three addons that help your early progression into instanced content in the game:
DBM - this addon helps you get used to the mechanics of a fight faster than the usual in game indicators. The game does not necessarily warn you if you have an important mechanic on you. DBM does, and you can even customize in what way it will warn you.
Weakauras - this is mainly for the tidiness factor. You can use this to set up where your essential skills are positioned on your screen, you can use it to track buffs that are important to you, or even track buffs that aren’t tracked by the basic UI. You can also get DBM-style customized warnings for specific boss-fights and mechanics.
Details - For tracking a bunch of useful information concerning damage, like what was your biggest damage source, how many dispels happen in a fight, how many interrupts happened up to a specific point in fights.
For trading TSM is a good thing to have, but it takes a bit to set up, so keep that one in mind if you give it a try.
Some people also like using UI addons like ElvUI to set up their UI the way they want to.
For gear tracking, atleast for raids, there is an addon called Method raid tools, which tracks what gear others are wearing. It also has other, useful built in functions like checking if everyone has enchants on their gear, a note function which you can use to set up rotations for handling mechanics as well as highlight a healing rotation, as well as to see what consumables people are using.
Not sure if retail WoW has a functioning addon for that, I use Auctionator in Classic so you could try that.
I would also suggest downloading Weak Auras and learning how it works. It bassicaly lets you track CDs, Buffs and Debuffs more efficiently among other things.