I’m here to give a few tips on how to get a decent amount of gold to get you started and most of the techniques will lead to millions/billions of gold. Technically they all do, but most are much slower than others.
The best thing to do while progressing through the game for the first time is to sell the items you find or are replacing. The best way to price your items is to go to the Auction House, right click the item you have under the ‘Sell’ tab, then clicking ‘Search for Similar items’. In all cases, except for legendary items and set items, it will fill in the 6 attribute slots with whatever is on the item. Be sure to remove anything that is not a Main Stat, Vitality, Sockets, Crit Chance, Crit Damage, Attack Speed, Experience and All Resist. You also want to enter a stat that is a bit lower than what is on the item. IE if the str is 35, enter 30 instead.
Those are generally the highest sellers, especially under level 60. CC and CD are probably the lowest sellers below level 40. The quickest way to sell these items is by undercutting everyone else. Some of these items can sell anywhere from 20k-500k. Be sure to collect all Blacksmith/Jewel Crafting Plans/Tomes. They sell for quite a bit now, especially since nobody collects them.
Be sure to grab all rare and blue items. Salvage them for their materials. Right now, the materials go for a lot of gold. 2k for the Exquisite Essence. Most blues sell to the vendor for 100-500 gold and at times, you can also get an Iridesent Tear which also sell for around 800-1,200 each.
Another few good items to grab would be Tomes of Secret, which can sell for 400-600 gold each. We also have all the Flawless Square Gems which drop quite frequently. They vary a lot, but rubies are the biggest seller right now going for 5,000-10,000 each.
These are by far the most lucrative ways of making money in the game. There is Flipping Items on the Auction House or buying gold from the RMAH. You can work for an hour and afford 200m gold. When flipping items, you just find the items with a bid lower than the lowest selling buyout of the same stats and type and attempt to win it. Always bid what you’re willing to part with. The best way to win these items is to bid at the last seconds of the auction. Good luck to you !