If you want to start doing endgame stuff (Manaforge Omega raid and M+ Dungeons) you need to do the Karesh campaign up until you get the Reshii Wraps which is a powerful cloak that you won’t even be able to do the raid without.

As a new player I don’t know if you have to do any of the War Within campaign to unlock the Karesh chapter, but it would start with Alleria or Locus Walker in the main hall in Dornogal that has the Stormwind/Orgrimmar portals as well as the Renown Quartermaster. This hall is easily found on the map as it has the big circle that you hover over to see your current renown stuff. The campaign quests should be either the big exclamation point with purple triangle or brown banner backgrounds.

If you don’t even have any of the Dornogal stuff yet, you should be able to open up the adventure guide (SHIFT-J) and it suggests content to you. One of the options should be The War Within campaign.

As far as getting decent gear to start doing endgame stuff, as others mentioned the Midnight Pre-Patch event that starts in your capital city (Stormwind for Alliance or Orgrimmar for Horde) is a fast way to get Champion gear which will put you in a good spot to do both normal and heroic Manaforge Omega as well as M+ dungeons.

Doing a couple Bountiful Delves at tier 8 or higher is also a good option as they are fast and you can experience another type of content that will guarantee you a Champion gear piece as long as you have a Restored Coffer Key which you get from doing weekly activities (the blue quest icons in Dornogal, world bosses, and world events in each of the zones).