There is a lot that can be said for your user-interface and is an area which can make a huge difference in performance but there is very little that I would consider mandatory for a first time raider, and if you have not used it before getting too many new things just before your first raid is going to cause your performance to suffer.
So, make sure you have the basics and then as you decide to raid more look into improving your raid user interface.
Boss Mod: This is essential to a new raider, and if you have never raided before I strongly recommend DeadlyBossMods as it provides more direction then BigWigs (the other option) and is better if you do not already know the fights.
GTFO: DeadlyBossMods covers some of this functionality, but GTFO covers more bad stuff on the ground.
Discord: Not an add-on, but if you are raiding in a Guild voice coms are almost certainly going to be available; you shouldn’t ever have to talk but it is very important to be able to hear the raid leader, and it is often much easier for them to answer any questions by speaking rather then attempting to type it in, even if you ask the question in text.
If you want to add some useful tools to your user interface and won’t be overwhelmed or have some extra time to get used to them. You should look into:
ElvUi: this is pretty much the consensus best-consolidated UI pack, you can find layouts and profiles on wago.io from many streamers.
WeakAuras2: there is a ton of functionality on this add-on as it allows for the sharing of many UI elements, from class rotation helpers to HUDs you name it, you can also find packs on wago.io. of specific note is the T21 WeakAuras collection.
ExRT: Exorus Raid Tools add a ton of functionality for tracking raid cooldowns, attendance, buffs, WeakAuras2 and more. Your raid leader almost certainly has this installed, it can be neat to check out.