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.
Special mention is the Darkmoon Faire Top Hat and WHEE! buff. While not always available due to the DMF being inaccessible most of the month, if you happen to be leveling when the DMF is running, it might be worth your time to go and get these. AFAIK they do stack with heirlooms, although I have not personally tested it. Fair warning: These buffs do NOT stack with each other, and the hats disappear after the DMF goes away later in the month(unless you only log in when the faire is running).
You earn a large rested experience bonus when you log out in this rest areas. This gives you a massive 200-percent bonus to experience gain from most actions (though experience from quests isn’t included). The amount of rested experience you earn increases the longer you’re logged off, and your experience bar will be blue (instead of purple) while you have the bonus.
Vigilance Perch is essentially an automatic mount when out of combat. Despite its tooltip, it only allows you to fly in flying zones, so you will not be able to fly in Battle for Azeroth zones. This is a great Legendary to pair up with Sephuz's Secret. Choosing between this and Apex will come down to personal preference and how you prefer to level. When speed-leveling with consumables, you will not get full value out of the mount ability of this Legendary. If you are just leveling normally, try them both out and see what works best for you.
Azeroth paid a terrible price to end the apocalyptic march of the Legion’s crusade—but even as the world’s wounds are tended, it is the shattered trust between the Alliance and Horde that may prove hardest to mend. As this age-old conflict reignites, join your allies and champion your faction’s cause—Azeroth’s future will be forged in the fires of war.
Prepare the Horde for war by recruiting the ancient empire of Zandalar. In this troll-dominated territory, ancient evil waits to be unleashed on the world as you battle crazed blood-troll worshippers, gargantuan dinosaurs, and titan constructs. Discover Zuldazar, the oldest city in Azeroth; unveil the bleak swamps of Nazmir; and traverse the deadly deserts of Vol'dun.
The most important step in being able to buy a mount and make other large purchases should be self-evident: saving. Economize as often as you can, and don't buy anything unless you absolutely have to. You can burn through hundreds of gold even before level 20 by visiting the auction house for new equipment at every opportunity. If you do so, over the long haul you will be left with very little to show for it. Before level 20, keep your eyes on the prize: getting that mount. The mount helps you move faster. Faster movement means faster killing, faster questing, faster quest turn-ins, and faster leveling. It is the most important tool to fast leveling you can get at level 20, and infinitely more important than getting your hands on that Left-Handed Vorpal Cleaver of the Zipswitch that you could have purchased at level 23. Stay focused.
Engineering is a crafting profession that primarily revolves around fun recipes. The gold-making potential from engineering is limited in terms of Legion recipes. The Skullblasters can be obliterated for profit. Engineering has a lot of profitable old world recipes, primarily mounts and various unique effect items and toys. This profession is best relegated to an alt unless you love the lore-aspect or have other sentimental reasons to keep it on your main.
Simply put, its about expectations. Transmog is not a get rich quick scheme. You won't wake up the next day to a mailbox full of gold on day one, two or three. For most of us, it will take months to get to a point where we have to refresh our mailbox to get to the rest of the gold. Our expectations are high, and the initial results are rarely ever in the same ballpark. A few days or weeks of less than stellar results and we give up, because we lack patience. If you want/need gold now, transmog is not the solution.
If you are dead set on leveling a production trade skill instead of taking two of the gathering skills, remember that not everything your trainer offers is worth buying. While it might be nice to have a long list of colorful shirts and dresses to produce as a tailor, for instance, the truth is they offer very little in the way of potential revenue. Also keep in mind that, generally, whatever items you craft at lower levels will not likely sell for more money than you could have made by simply selling the raw materials used to make them. For this reason, two gathering skills are highly recommended until you get closer to 80 (70 if you don't have the WotLK expansion or 60 if you don't have the TBC expansion).
I recommend not counting on transmog sells for your monthly gametime. Its a great market, but also pretty random. You have to have the right items posted at the right moment if a player needs a certain item. If this is not the case, of course you will not sell anything to him. There are people making millions of golds a month with transmog. But they are very professional at it and it really takes a lot of time.