The quickest and easiest way for you to start making gold at 90 will be dailies. You get gold, reputation, coins for rerolls, and valor points for gear. Beyond that, you will need to learn your servers economy and auction house.
The first thing you need to realize with making gold with your professions, is that nothing is free. So in most cases, you will be better off skipping the gathering professions and leveling dual gathering professions. Though for getting started, you might want to stick with a gathering / crafting profession combo until you are comfortable playing the AH.
Inscription, paired with herbalism, has been the most profitable crafting profession so far this expansion. Once you hit 600, you can start making scrolls of wisdom on a daily cooldown. You then convert those scrolls to DMF cards, ideally creating decks which can be traded for 476 trinkets during the DMF. The process mainly involves buying cheap end-game herbs off of the AH and milling them into ink of dreams which can be traded for starlight ink. While one of the easiest professions to level, the return on cards is constantly on the decline.
Jewelcrafting is the next big gold maker this expansion, and should overtake inscription, though there is a lot of competition. It goes with mining, prospecting mass amounts of ore for gems to be cut or fashioned into jewelry, though it is best used as a "gathering" profession for enchanting, the third biggest gold maker imo. Your best bet would be to Google the JC shuffle. While not as strong as it was in Cata, the shuffle is still quite profitable.
You could also try leveling alchemy and herbalism again, as alchemy barely edges out inscription for ease of leveling. There is some profit to be made in transmuting primal diamonds for jewelcrafters as well as rare hybrid gems if you can find cheap golden lotus. Or you could take the elixir mastery route and try turning a profit with flasks. Finally, there is potion mastery, though the market seems poor on my server.
For a new player, I would recommend avoiding the armor crafting professions. I have done well with leatherworking this expansion, but it is really a niche market. I would also start out with a "simple" auction house addon like Auctioneer. It will simplify posting your goods and researching what you might want to craft. It also comes with enchantrix for auto-prospecting, disenchanting, or milling depending on your professions. For more advanced tools, investigate Trade Skill Master, which has a steeper learning curve and the Undermine Journal website for your server.