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.
If you can answer all of those questions with yes, then you are in a position to make a lot of gold! You can continue doing every step to get to the gold cap as quickly as possible. The security achieved through this can buy you virtually anything you desire in the game, including achievements that would otherwise be impossible on your own, which leads us to the final gold making tip - Selling achievements. If you are the leader or officer in a guild, you can take good acquaintances on your raids with you for achievements like finishing ICC or any other hard raid achievements that your guild finds easy and manageable. I have seen achievements go for 20k+ if you find the right client! This is very situational, however, and is by no means necessary to hit the gold cap, it just helps!
Just before Battle for Azeroth goes live, you will want to log all of your characters out in the zone of Silithus so that you can acquire the quest to talk to Magni and obtain your Heart of Azeroth (refer to our Heart of Azeroth guide for more information about this item). This is the new artifact-type of piece of equipment that you will use for all of the Battle for Azeroth expansion. Once you acquire your permanent neck slot piece of equipment, get back to Stormwind or Orgrimmar to begin your scenario that leads into the introductory areas of Kul Tiras and Zandalar.