First, what kind of raiding progression are you interested in? This is important because it also determines the amount of work that you have to do. I’ll split it into three different categories:
The casual raider: you wish to fully clear LFR/normal. You don’t need to know a lot about the game, you don’t need to be good, and you can just focus on playing the game however you want and you’ll reach these goals. Normal will be noticeable more difficult than simply LFR, but these are both objectives that one can accomplish with relatively little effort.
The curve raider: You wish to complete the raid tier fully on heroic, earning you the Ahead of the Curve achievement. Pretty much the milestone for any raider who just does pugs and isn’t in a progression guild. For this you do need to have at least a basic understanding of your class and it’s mechanisms, and on top of that you need to know the fights well enough not to screw up. To achieve this completely solo, you need to put in at least a moderate amount of work.
The cutting edge raider: Clearing the tier on mythic earning you the Cutting edge achievement. Now this is the hardest one, and to succeed you first need to find a guild and prove yourself to be a very capable player to have any real shot at joining a cutting edge raiding team. This requires commitment and time, and you need to have a very good understanding of your class and the boss fights. Reaching this level will take time, and you definitely need a lot of experience before tackling this one.
As per usual, I’ll guess that you want to be at least average, meaning that you’re going for ahead of the curve. So, what do you need? Gear, skill and knowledge. How does one go about obtaining those? Well, for starters decent gear can be found pretty easily at this point, you can do Nazjatar/the new 8.3 stuff and already be on your way to +430 ilvl. After that, you can either do LFR/normal for further gear, possibly some m+ or maybe even horrific visions which you can also do completely by yourself. By the way, this includes getting your heart of azeroth level up, at least to the point where you have unlocked all the essence slots (currently that’s neck level 75). You can do this quite painlessly by doing the daily AP wq’s without forgetting about emissaries (also remember your weekly Island Expedition cap). Now with 8.3 out there’s a lot of new systems in place, so you’ll need to do some playing to get yourself back into the fray. Worry not, it won’t be too difficult if you just play the game. (If you want more information on this in particular, ask).
Next, you would go about obtaining your skill. Well, might not sound as easy, and it probably won’t be if you’re not experienced. WoW is a skill as any other thing, and experienced players will be able to pick new specs and already be better at them than the average player who’s played that spec for years. How to get to that point? Get serious about improving, and try to focus on one spec at a time. I’ll give you some resources and hints. https://www.icy-veins.com/wow/class-guides beyond that link you can find a bunch of class guides for any spec. Pick the one you’re interested in, for example unholy death knight. After that, give the page a good read and try to really understand what’s going on. Nailing down your rotation is the most important thing. If you use that as a reference, you will learn the basics of any spec, and in most cases that is good enough. However, if you’re getting serious about getting good, then you’ll need other resources as well. Here’s two: class discords and warcraftlogs. As for the former, if you’re at all familiar with what Discord is or how it works, there are a bunch of servers for different classes that are there to help you become a better player, and they will have resources + really good players in them to help you with your questions (from my experience, the top players of each spec tend to be in those discord servers). Here is the link for the death knight one, if you’re interested: https://discord.gg/acherus. Note that icy veins will be enough to get you started, so you don’t need to join this discord immediately, but it can be a good thing to familiarize yourself with it (resources are usually pinned on channels).
Now for the latter, meaning warcraftlogs.com . I don’t know if you’ve heard of it or not, but it’s essentially a site that tracks your performance in raids, if someone you’re raiding with happens to be “logging” that particular raiding session. You might find yourself there even if you had no idea that it existed if you’ve raided before. The site will also give you a score based on how well you did compared to other players of your spec, and there are color codes to go with it as well. Grey means that you either died/are extremely low geared/really really bad. Green usually indicates a lack of understanding of the basics/bad gear. Blue is average but also not really good, this means that you’re either kind of poorly geared or just not very good at your spec. Purple is kind of good, this means that you probably know the basics of your spec at the very least and have decent gear. Orange is very good, this means you know your spec and have competitive gear. Pink is extremely good, and this is just orange but even better (pink is what I usually aim for). Lastly you have gold, and this means that you hold a top parse for a given time, meaning that you were the best player of your spec at some point in a fight. I’ve had these too, but they’re not something that you’ll get unless you’re really going for them. Essentially having a gold parse means that you’re technically the best player in the world for that given moment on that given difficulty on that fight. And the number that comes with it indicates the percentile you’re in, so if your number is 20 it means that 80% of players did better than you, as you’re in the 20th percentile. However, if your number is 99 (pink) then it means that you’re in the top 1% of players.
NOTE: this is a tool to track your own skill level, and a little bit more advanced. Again, focus on the basics first. The reason I’m giving you this information is because it is crucial at the point where you’re not sure what you could do better anymore, and to finding your own mistakes. You can look at the best players for your spec and see what they did instead/differently. There are also analyzing tools that will point out mistakes in your rotation if you have any that work with this site. Wowanalyzer.com is one of them. Again, not necessary information right off the bat, but will be extremely helpful in the long run.
Lastly, knowledge. Well, this just means looking at boss guides and to really understand what’s going on during a fight. There’s a bunch of guides for every boss, if you’re doing anything above LFR I recommend you take a look at the bosses before entering the raid so you’re not completely clueless (LFR alone does NOT prepare you for difficulties above it). I’ll also mention addons here. At the very least, get a damage meter and a boss addon. A good damage meter addon would be Details!, while for a boss addon you could choose between DBM (deadly boss mods) or Bigwigs. DBM is probably more beginner friendly. There’s a lot more useful addons that come in to help you at later stages, such as Weakauras 2, but that can be a little intimidating from the get-go, so get comfortable with the basics first (and you don’t need weakauras until you understand what you need it for).
Also, sim your gear. You can use raidbots.com for this. It’s pretty straight forward, you can look for your character over there and sim a piece of gear against your current equipment for example and see which one is better. Extremely important that you use this to determine what items are good for you. Again, this is an advanced tool, however I recommend that you use it as soon as possible because it is not difficult and will help you immensely later on, even as a beginner, and once you get good this will be one of the most important things.
TLDR: use https://www.icy-veins.com/wow/class-guides to learn about your spec. Watch boss guides before doing raids. Download a damage meter and a raid boss addon. Also, read the rest of the post because it is important information if you want to improve. Not all of it is necessary right now. Reply to this message if you have further questions or need clarification on something.
PS: while this post is long, I did leave a lot of stuff out and barely scratched the surface. Don’t be intimidated, do the stuff in the TLDR at the very least and go from there.
