Level 40 in vanilla WoW is known as being sort of the halfway point to level 60 and when you finally can get your mount. The mount costs a total of 100 gold, 80 for the mount and 20 for the training. You’ll need to buy the mount from your racial vendor, unless you’re exalted with that faction (i.e. Humans get horses, need exalted with Darnassus to get tigers, etc.).
Best Advice: The first thing is start planning for your mount at level one. Don’t spend anything at all ever. Avoid the auction house except to offload items. There isn’t very much you can buy at lower levels that’s worth the money and other players will gleefully spend their gold. Don’t take a flight path every single time as well. Flights are expensive. If it’s not across the entire continent, just walk!
Grinding: Grind while questing. Just farm enemies while walking to places. Even after the quest is done just keep grinding away. Loot everything.
Gathering: Gathering skills are super important. The more you can gather and sell, the more money in your pockets and the more likely you’ll have that 100 gold saved up at level 40. Skinning + mining/herbalism are STRONG for making money.
Compare ores vs. bars on the auction house. Ore often sells for more due to folks wanting it for leveling.
Class Abilities: Don’t buy every ability unless you use it. Like for instance, you don’t need pummel for leveling and you can leave it at rank 1.
Gear: If you do want to spend money on gear, focus only on your weapon. Quest for everything else.
Discounts: Phase 1 won’t have PvP ranks, but rank 3 gives you a mount discount. So does being honored, which you should always follow the quest path for your race unless you’re using a leveling guide.
Economically Established Servers: Fishing Oily Blackmouths in the Wetlands is a good means of making money once the economy starts to take off in WoW Classic. Same thing with a lot of odd crafting materials. Won’t work on new servers. For instance, before an economy is established you may want to vendor leather. Later on, it may sell for way more on the AH. Be sure to do your investigation.
That’s about it. Just don’t spend your money as you’re leveling except on repairs. Sell what you can to the vendors and the AH and if you hit level 40 and are still short: ask friends.