Congratulations on reaching Level 120! There are a myriad of new things to do. The first thing you should do is head to your faction hub ship where you have been doing your mission table quests and footholds to pick up and complete the quests necessary to unlock World Quests. World Quests will provide you an opportunity to grind reputation levels, acquire gear, and gather Azerite. There are tons of new things to do that we will cover in our Level 120 Battle Guide found here: Level 120 Battle Guide.
World of Warcraft offers a lot of ways to make money (gold). There is no one "right" way to make money, although there are some definite wrong ways! Some people like to play the Auction House, some people do their daily quests, some people tend to farm, etc. Many characters do a combination of all three of these along with other activities. Below is a compendium of money-making methods.

 Kul Tiran human (8.1.5) - "Kul Tiras was founded by fearless explorers who sailed uncharted waters in search of adventure. As a key member of the Alliance of Lordaeron, the Kul Tiran navy dominated the seas of Azeroth with its legendary fleet. But after years of turmoil and hardship, the kingdom grew isolated and vulnerable to dark influences. Thanks to the help of brave heroes, House Proudmoore has been restored and Kul Tiras is poised to fight alongside the valiant Alliance once more."
2. Once you have an interested buyer, stay firm in your price. This does not mean to never make them feel like they are getting a bargain. You can subtract 5% off the price and not be hurt as long as you still make a profit! Remember this, as that 5% may not be a big deal to you, but the buyer will feel special and might buy the item for the slightly reduced price. I can not emphasize enough that this is not the same as undercutting! Do not drop every good every time or you develop a reputation as a soft seller and people take more liberties in their dealings with you. If you give someone a constant 5% discount, then they begin to want 10% and then 15% until you no longer make a profit. Discounts are used when you feel that the seller may be pulling away and you really want to just profit and move on!
A cool little pet farm that can take minutes or ages depending on the god of luck.  This spot contains some of the most dense troll spawns in the game, and each has the highest chance to drop the parrot pet thats soo sought after.  On every serve rthis per has  amedian value of about 70,000g so picking one up can be a pretty cool feat.  Like the sprite darter hatchling spot its, a random farming spot that requires alot of RNG to pull off, but if you get it to drop you can make out in spades.  The other drops are miniscule when compared to this spot.
While I don't agree with some of the story choices as of late, BfA is a great addition to the WoW expansion list. Directly following theWhile I don't agree with some of the story choices as of late, BfA is a great addition to the WoW expansion list. Directly following the explosive conclusion to Legions End, we see the two warring factions at each others throats once again. In addition, we are seeing a trickle of new races becoming available with unlock requirements. As of patch 8.1, the grind for these races has been massively alleviated.

To use it as a bank, figure out how much you want to have on-hand on your character based on how much you normally spend on repairs, food, ammo, etc. and send the rest to the bank alt. The principle here is "Out of sight, out of mind.". Money "you don't have" cannot be spent, requiring you to log out of your character, and then to log into the alt.

This new farm takes advantage of the level scaling changes implemented by blizzard.  Event NPC's like the NPC's found in the twilight outpost, scale up or down depending on your level. HOWEVER, your level also determines which loot tables each mob will have.  By using a level 58 character, you can successfully loot items in the 57 – 59 item range, a range which has some of the most expensive and well sought after transmog items in the game.  In less than 30 minutes farming this spot I was able to snatch up some glorious legplates[/URL] (on video) – worth a stunning 150,000g.  Watch the video for absolute details I suppose, but the gist of it is you need to create a level 58 DK, go to the twilight camps, and start the killin'.  Unlike before, an implementation of a daily event has changed the loot tables!   This spot is the new hot thing on the block.
The most simple thing is TradeSkillMaster. This AddOn shows you quite an amount of information in the items tooltip. In my opinion, DBRegionMarketAvg the best source. It tells me the price, an item was posted on all EU realms in the last 14 days. Why not DBMarket? DBMarket only shows you the items value from your server in the last 14 days. This value often gets manipulated. Some people start to post an item with a 500g value for 500.000 Gold on the auction house for several weaks. As a consequence, DBMarket is rising towards 500.000 Gold.  If you use DBMarket with 100% and post your item for 500.000 Gold, nobody will buy it.