Once you are level 21 or 22, you can head over to Kalimdor and the Stonetalon Mountains. The entry level quests towards the south entrance (concerning the Tauren) may be to low to be worth it, so head north to Sunrock Retreat and grab every quest that you can. Once you've completed all of them make sure you are at least level 26 before we head to the next zone. Grind it out to level 26 if you need to.
Elemental items (Primals, Eternals, etc.) can be a good source of money, because they are always in demand on the AH. See which mobs you will most benefit from farming, then set out for a few hours. In the process you will also most likely collect significant vendor trash, and may get other profitable item drops as well. Note, however, that older elemental drops usually lose profitability compared to newer ones, i.e. Primal Water sold well to level 70 players in BC, but sells far less well to level 80 players in WotLK, because level 80 gear requires Eternal this'n'that.
Buffs. Food/Flasks/Scrolls/Potions/Elixirs are a small, but noticeable, buff to your killing speed and survivability. If you have a max level character that can already supply you with cooking or alchemy, you'll be very well off, because the AH is a cesspool for this. I can't really give you specific foods or potions to look for, since each AH is going to have different supplies. Generally speaking, lower level foods are going to only last 15-20 minutes until you can buy the more powerful versions at 60+. Look for anything that adds versatility or your primary stat, and buy stacks for each level range(1-60/70/80/90/100).
Herbalism is also a good source of money. Herbs are required by alchemists and scribes. Unlike ore which is found only in rocky areas, herbs can be found in many places. Be sure to check on the auction house for what herbs are in demand for a high value; often a lower level one is very valuable, so you can farm an area you already know for quick money.
Set sail for the previously unmapped isles of Azeroth. Battle in groups of three as you race against cunning rival intruders—or enemy players—to collect the island’s resources. Constantly evolving challenges await as you traverse frozen landscapes near Northrend, open the gates of an abandoned Gilnean castle, navigate a war between elementals and more. Tides of Vengeance adds two new Island Expeditions to the mix and two new random events, as well as updated rewards.

I dont know if this exist already, if it does I couldnt find one, but it would be nice to have a guide with the location of all daily quest broken down by Alliance, Horde, Faction, Neutral, etc. I know there can be variable quest for things like Fishing and Cooking but just having the location for those quest givers would be great. If it were a stand alone addon that would be even better!

Do quests and complete the story of your first chosen area before returning to your capital to scrap all of the equipment items you acquired. You should be around Level 113 by the time you complete your first area. Before selecting your second zone, head to your faction hub ship to acquire your introductory quests to the mission table and your champions. Complete this introduction and then pick up the quest to establish your first foothold in enemy territory.
Alpha Beta PTR Classic 1.1.0 1.2.0 1.3.0 1.4.0 1.5.0 1.6.0 1.6.1 1.7.0 1.8.0 1.8.4 1.9.0 1.10.0 1.11.0 1.12.0 Burning Crusade 2.0.1 2.0.3 2.0.4 2.1.0 2.1.2 2.2.0 2.3.0 2.4.0 2.4.3 Wrath of the Lich King 3.0.2 3.0.3 3.0.8 3.1.0 3.2.0 3.2.2 3.3.0 3.3.3 Cataclysm 4.0.1 4.0.3a 4.0.6 4.1.0 4.2.0 4.2.2 4.3.0 4.3.2 Mists of Pandaria 5.0.4 5.0.5 5.1.0 5.2.0 5.3.0 5.4.0 5.4.7 Warlords of Draenor 6.0.2 6.0.3 6.1.0 6.2.0 Legion 7.0.3 7.1.0 7.1.5 7.2.0 7.2.5 7.3.0 7.3.5 Battle for Azeroth 8.0.1
If you bought Legion and have no desire to go through old content, good news! The new expansion comes with a free boost to level 100 for one character. You can buy these boosts from the in-game store whenever you want, but they normally cost $60, so savor this free one. Use it for a class that you are confident you will enjoy. As a fresh level 100 character, you will be able to immediately begin the quest that takes you the Broken Isles, the new region where Legion takes place.

Okay, this farm really picked up steam here in late legion.  Generally what we're looking for is volatile air, and other volatiles when combined with the potion of treasure finding.  This farm is as easy as pulling the entire instance all the way to the first boss – and AOE'ing down ebery mob that follows.  In the video you can see me getting volatile airs in stacks of 11 for each pull I make.   Wit the trending price of each volatile being worth at least 100g, I was making 1100g per pull.  It's by far the most effcient volatile spot, but it has hits limits as you can only reset an instance 10 times before you have to wait a while to do it again.  If you only going to be on for an hour or so, this is the spot you want to hit.  It's great, fast, and awards a high liquid value of items.  Awesome Stuff.
Alchemy is a great profession for gold-making as the main outputs are consumed on use, this means that there will always be demand for new potions and flasks. The old world recipes are also very profitable. Sadly I don’t have Legion alchemy so I can’t personally help you. The best guide I was able to find on the internet is this video guide by Ninja Kuma.

I do not recommend going for TBC, although I remember Hellfire peninsula to be decent, everything else is rather slow. Instead go for WotLK, Borean Thundra gets you levelled very fast, and then you can head straight for Icecrown at 67, which is also insanely fast, although admittedly has a few elite quests. You can go to Sholazar Basin alternatively. 2 zones should get you 60-80 no problem.
Most of the players make the mistake of only listing 30-50 transmog items on the auction house and then hoping to get half of them sold until the next day. Me and a lot of other gold farmers and transmog sellers do have 3.000-10.000 items on the auction house across various realms. It does not mean you have to do it that way – but if you have 1000 items listed, chances are higher you can sell up to 10 per day. With only 100 items listed, you have to be lucky to sell one. Of course, the sells are not guaranteed if you post more items. But i’m sure you have a lot more fun if you open your mailbox and 10-20 items are sold