Here is an updated list of resources for Legion and beyond!
Gnome Discord: https://discord.gg/dW9PW
Both of these sites have excellent class guides. This should be your first stop after patches and any changes to your class to see what new info you need to know.
http://howtopriest.com/index.php - If you are a priest, this is the go to site for all things Holy, Shadowy, or Discipline...y.
https://www.altered-time.com/forum/ - Mages, same deal here at Altered Time.
Death Knight: https://discord.gg/0ez1cFfUH3ingV96
Demon Hunter: https://discord.gg/zGGkNGC
Class Discords are some of the best class specific info in the game. Much of the time there are high end theorycrafters hanging around to answer questions. Read the pins in you spec's channel or the FAQ board to keep up to date on your spec.
Emerald Nightmare - http://www.wowhead.com/news=255999/legion-survival-guide-emerald-nightmare-mythic-dungeons-world-bosses-legion-seas
Trial of Valor - http://www.wowhead.com/news=256933/patch-7-1-new-raid-trial-of-valor
Nighthold - http://www.wowhead.com/the-nighthold-raid-guides-and-rewards
https://www.kremlingkrew.com/heroic-nighthold-guide/ - The drawings are relevant for both heroic and normal, and the strats can be condensed for normal, very well layed out.
Raids and Mythic Plus - http://www.icy-veins.com/wow/raid-and-mythic-guides
Read up on the fights, get an idea of boss mechanics, and loot here!
Fatboss - https://www.youtube.com/user/FatbossTV/videos
Two Minute Tips - https://www.youtube.com/user/losgamingshow/videos
Coaches Corner - https://www.youtube.com/user/DeathJestersGuild/videos
Kelani - https://www.youtube.com/user/SignsOfKelani/videos
Use the videos here to get an idea of the content we will be raiding.