Welcome back!
And yes, I believe you are a new player. 
Dottie gave a great definition of someone who has passed being a new player once that I never forgot: you stop being a new player when you have levelled to max in one expansion, and then levelled to max in the next. The key is the understanding of what end-game is, and seeing the change in end-game between expansions.
And on to your question …
You can play with the default UI and no addons. It is definitely not optimal for efficiency, though. Different addons do different things for you. Some make pretty sweeping changes to your view. Some make pretty sweeping changes to your gameplay. Some don’t really affect either.
The main thing I would say to a new player who hasn’t used addons before is to add addons one by one. If you try to change too much too quickly, you WILL get lost. You will not know what that flashing thing is, or which addon it’s coming from. You will not understand why sounds are blaring. You will accidentally do things you didn’t mean to do.
Some addons are simple. Some are really complicated. One - a very useful one - may mail all your gold to a stranger when you visit a postbox, if you have accepted the wrong program for it. (I think Blizzard have prevented this happening now.) Further, some addons interfere with each other, throwing distracting errors. And of course, many addons break when Blizzard changes the game in new patches, meaning you need to update or reinstall them, and reset their configuration.
Mostly, addons are fine, pretty much everybody uses them, and they are a major part of playing the game. But for each addon, you should know what it does and why you want it. Installing a bucketload of addons all at once when you’ve never used them before is a great way to get confused and frustrated.
And then we have complete UI kits, like LUI and ElvUI. They provide pre-defined screen layouts. Personally, I hate them, so I am not the best person to advise you. I tried ElvUI once, and spent far too much time fighting with it to try and put things exactly where and as I wanted them. However, for people who like that kind of thing, that’s the kind of thing that people like that tend to like. 
It’s entirely up to you. If that LUI look really appeals to you, go for it on its own, and get used to it. Then start adding addons one by one, setting up and understanding each as you do so.
Remember, you can at any time remove an addon by removing its directory in
…World of Warcraft\_retail\Interface\Addons
and you can disable it by clicking on the Addons button in your character login screen and unticking its name
and you can configure it with
Esc->Interface->Addons tab.
We haven’t had an add-on thread since the forums were changed. Here’s one from the old forums you could have a look at:
https://eu.battle.net/forums/en/wow/topic/17615422038
I’ve edited it slightly to update it
We haven’t had an addons thread in a while. Looking at the top downloads list on https://www.curseforge.com/wow/addons will give you the general idea.
Some Addons I like are:
Deadly Boss Mods (DBM) and its modules for raid and dungeon tactics
Skada for damage and healing meters
Bartender4 for managing your bars
Threat Plates for nameplates
SellGrey for selling junk
Quick Repair for auto repairs
Carbonite for maps. You’ll love it. Or hate it. Or both.
Auctioneer, Auctionator and Trade Skill Master for the Auction House
Altoholic if you have many characters
GTFO reminds you when you’re standing in the fire
Pitbull gives you nice unit frames
Postal is an improved mail interface
You don’t NEED any of these, and there are many others, some generally useful, some specialised.
Ah, here’s a post I made
26/08/2016 21:21Posted by Gráinne I recently reinstalled all addons from scratch - just adding them one by one when I felt I needed. I haven't "done" anything at all since the pre-patch. Here's my current list of addons: General: Addon Control Panel Advanced Interface Options Bartender4 Altoholic DejaCharacterStats OmniCC Prat Sell Grey Items Maps: Carbonite 7.0.0 HandyNotes (with Broken isles module ready for next week) Auctions: Auctionator Postal Pet Battles: Battle Pet BreedID Derangement's Pet Battle Cooldowns Rematch Fighting: Classtimer DBM Focus Interrupt Sounds GTFO HealBot PitBull ThreatPlates Skada As I look down that list, I say "Why don't I have....." quite a lot. Quick Repair, TellMeWhen or WeakAuras (I change between them from time to time), TSM. More, I'm sure. Some of them weren't available at the start of the pre-patch. Some of them I just haven't felt the need for yet, since I haven't been doing anything more than derping around and doing some invasions.