The World of Warcraft API has a list of functions that you can interact with and to talk with WoW using Lua. These are your building blocks for manipulating WoW windows and chat boxes and so forth. Looking at these for the first time can be very daunting. But don't be discouraged. After getting more familiar with Lua the WoW API gets much easier, and the documentation gets much easier to read. Keep in mind there are many different ways to accomplish your goals, so when going though the API keep looking until you find a way that best suits what you need.
I've been having what is apparently a scaling issue caused by inflated secondary stats on some Wrath of the Lich King items. In particular, my Highmountain tauren warrior leveled through Northrend and picked up a green, iLvl 87, 2h sword. All of the secondaries were in the +25 to +30 range, and the Dugi Gear Advisor decided it was better than the heirloom 2h axe I was using. So I went ahead and equipped the sword. 

Level with a pet if you can because it will help keep you safe. If your pet is tanking all of the damage from the enemies you’re fighting, you won’t have to. The other reason is that pets are easier and cheaper to heal than you are. If you don’t have a pet, make sure you know how to use your defensive abilities. You can’t level quickly if you are dying all the time.
Excess Potion of Accelerated Learning: Acquired in your Garrison (part of the Warlords of Draenor expansion), this gives you a 20-percent bonus to experience from killing monsters and finishing quests. It lasts one hour, but you can buy it multiple times and use it back-to-back. Alliance players buy it from Sergeant Crowler, horde players buy it from Sergeant Grimjaw. Does not work above level 99.
Certain locations in World of Warcraft – primarily inns, capital cities, and private instanced zones like your garrison or class order hall – are rest areas. You know you’ve entered one because your character portrait will gain a “Zzz” icon where your level is normally shown, and because you can immediately log out of the game instead of waiting 20 seconds.
If you're looking for simple, just get actionator. Set it to undercut everything you post for 1c and alt click from your bags. It will place the item on the ah window, do a scan for that item and autoset the price for how much you told to undercut (you can change it accordingly if you feel like it). It will remember if you do stacks for which items and replicate it everytime.

Last I wanna share a curious bug that almost freezes my game and force me to disable the addon. Anytime I add a (Citadel Crossbow [of the Fireflash]) my fps go from 75 to 22, and It's on a paladin, paladins dont even use crossbows. I noticed everytime I put something in my bags the addon takes the math and my fps goes from 75 to ~22 for a while (3 to 7 seconds) and is think it's internal and also think it's fine.
Endlessly queuing the same dungeons over and over again can very quickly turn you off from the leveling experience. This can be counterproductive to your desire to reach 90 as quickly as possible, if you need to take extended breaks to cope or if you stop leveling altogether. God forbid, you may also make ridiculous threads on the WoW forums QQing about the lack of variety in the leveling experience.
As the first major content release of Battle for Azeroth, patch 8.1: Tides of Vengeance is adding a number of new ways to gear up, as well as increasing the item level of rewards from existing activities. Tides of Vengeance goes live on December 11th and will add most of the new content, but Mythic+ and PvP Season 2 will not start until the new raid, Battle of Dazar'alor, releases in January, at which time the item level thresholds will increase to the new Titanforge cap of 425.

after leveling up im considering leaving the game, plenty of quests with bugs missing npcs and really no one in the worls, feels like a single player game. also current content is the worst possible, leveling in legion remind me of how good it was, all the spots had something i could do, i look i farmed dh hidden appearance here, bfa its like wod with 0 content and nothing better then grind.
×