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.
The guide should advance to the point where you are in the zone's progression. There are a few steps that can-not autocomplete and they should (hopefully) have comments to the effect of -- you need to manually check this step off. -- For some reason you have to click these steps off 2-3 times to work. (the first time will move the tom-tom arrow, the 2nd time unchecks it and the 3rd click advances the guide).
Welcome! While this leveling guide is specifically made for the class and specialization(s) listed, we also have a general Battle for Azeroth leveling guide that focuses on the specific features of the Battle for Azeroth expansion including preparation of your mission table that will be used to unlock new areas, progressing on your faction's War Effort to save time at Level 120, and a reminder to keep ALL of the gear you find for the Scrap-o-Matic 1000 or Shred-Master Mk1, which will greatly increase your resource generating capabilities. Please visit our 110 to 120 leveling guide for this information and more (including Heirloom and War Mode advice that also applies to players who are not yet Level 110.
The waypoint directional system helps you to easily spot and reach a quest NPC or target. The step-by-step guidelines give you an idea of the materials you will at later stages. This minimises the time wasted while backtracking. Your quest progress and level will be updated automatically by the guide. This feature makes it easy to use any character. If you want to take a break or switch to battlegrounds or dungeons, you only need to re-tweak the guide to match your need at hand. Zygor Guides also come with flight path usage, turn in and quest accepting.
Allied races aren’t much better. You can only earn them by grinding out reputation. It doesn’t take that long but casual players will need to invest a couple weeks primarily to it, and focusing on that grind will take you away from others, like earning gear. Each race has its own grind, but the things you do to work through it remain the same, adding to your boredom.
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.
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.
As others have said, there are free alternatives... which is important to consider if the price of the Zygor Guides are a concern. If speed and convenience are more important, then they're worth it. Zygor will take you through the quests in the most efficient way possible, and their customer support and the speed with which they update the guides when changes are made to the game is impressive. I know I went from 80-85 a heck of a lot faster (in /played time) than my co-worker who doesn't have the guides. And we had a bet going to see who would make it first. For me, it was worth it. Your mileage may vary...
Some players even turn this into their profession by systematically "plundering" vendors in the game world and then selling the items on the auction house at a significant markup. The reason why this works (even for items which are on unlimited supply at vendors) is, that many players don't want to spend time traveling to specific vendors to get hold of a recipe or skill book. They would rather pay a slightly higher price at their local auction house. In some sense, they use the auction house as a "super market" or "convenience store". So it is completely reasonable and legitimate to be the supplier for this convenience store and make money out of it.

Once your character makes it to Outland and beyond, cashflow frees up considerably. The quest rewards are much better than in Azeroth. In fact, a typical character will earn from 1000-1200 in quest rewards and vendor trash while leveling 60-70 in Outland, and perhaps 1400-1600 from 70-80 in Northrend. The tendency is, therefore, to spend more freely after one hits 60. However, it is important for players not to go crazy on their spending once they make it to Hellfire. One thing is, training costs, repair costs, and consumable costs are also higher. More important, there is a large purchase that you are going to want to make at some point after level 60, your first flying mount and skill. The "bird" costs 40, the skill will cost you far more. Not only that, but if you want to fly it in Northrend at level 70, you'll have to shell out another 400 for Cold Weather Flying. And for those characters who will be 'farming' herbs or ore in either Outland or Northrend, an elite flying mount is almost essential, as it helps you gather almost twice as fast. That's another 5000 you'll be looking at. Therefore, budgeting carefully during the 60-80 leveling process is essential to ensuring you have sufficient cash on hand for making those purchases. Saving your pennies early makes that bird appear that much sooner.