I am looking to start doing dungeons and raids, and is looking for ways to help me prepare for mechanics, besides just failing them until I know them by heart.
The first option is of course video guides, but are there any in-game addons that inform you of the mechanics of mobs and raids? I know of Deadly Boss Mods, but they are more reactive: “move out of this NOW”. I would really like to be able to see the mechanics described before the fight, so I’m prepared. So each mob or boss I encounter, I get a recap of what it does.
There was one example, DungeonTips, but that looks dead today
The dungeon journal tells you what the abilities do. But its just a laundry list of “stuff” until you’ve seen it.
Youtube vids with quick summaries (as opposed to quite indepth encyclopedia vids) can be useful for an overview (just search for 1 minute wow boss guide for xxxx) and you should be fine.
I really like this weakaura, its a timeline of whats about to happen so I can see ‘x’ ability is happening in 5…4…3…2…1 and prepare.
https://wago.io/RaidAbilityTimeline
Addons you NEED are definately dbm or bigwigs not both though 
A rotation addon such as hekili will assist should you change your class or spec i also use grid2 for healing as i prefer active healing others such as healbot work too and possibly a ui changing addon i personally use zperl but thats to pretty up your unit frames only. If you want a damage meter there are a few options i use 2 at the same time combined with a threat meter addon.
The key is find what you like best some ppl minimise everything so they maximise screen area to see attacks and so on. Others like myself emphasise ease of use with frames its really your own prefference. An addon such as weakauras can literally do it all via plugins, be wary though bc some links are bad and can get you a virus i reccomend sophos x to scan the link when you press/click it.
Most are safe but all it takes is one bad one and boom your infected.