Perhaps the easiest way to earn money after you get your Chocobo license (or even, as you are trying to get it) is to collect Gausebit Grasses from Wadi Hares in Dangruff Wadi and sell them at the Auction House in Jeuno. These grasses are dropped fairly regularily by these hares, and, past level 18 or so, the hares and surrounding area should be pretty easy to you. If your attack and defense are high enough (or you have area attack spells) you should be able to take on multiple hares at a time to increase their drop rate.

The grasses sell typically for 1000 gil each at the Auction House in Jeuno, and they tend to sell quite rapidly. The reason for this is simple: Around level 20 people start migrating to Jeuno to get their Chocobo license. Enroute they discover they have some very tough areas to pass through, and often they arive at Jeuno by the skin of their teeth. Unless they plan ahead (and, let's face it, most people do not) they will find themselves without grasses for the Chocobo license quest, and stranded on an island of safety in the middle of a sea of impossibly hard aggro mobs. Often times they will feel it's not worth the effort, and just buy grasses from the AH or from Bazaars in Jeuno. This means there is a high demand for these grasses, and since most people coming into Jeuno don't bring ample supplies of grasses, this means there is also a low supply. High demand + low supply = profit!

So, fill yourself up on grasses at Dangruf Wadi and head out to Jeuno. It helps if your AH sales are empty so you can fill all slots with grasses. Stick whatever remaining grasses in your bazaar (at a discounted price) and sell everything there. You may have to wait a few hours to sell everything, because you will find sudden rushes of purchases every hour (typically around 0:00 in game time, for some reason).

If you already have a Chocobo license yourself, this is a very easy and quick thing to do. Basically spend your time travelling between Jeuno and Dangruf Wadi, collecting grasses and selling at Jeuno. Spend a few hours doing this and you can raise quite a bit of dough very easily.

One thing to be careful of in Dangruf Wadi is the gobs. If your level is high enough, then they wont be a problem, but around level 20ish you should be careful of aggro and linking (individually you should be able to take them, but get a few attacking at once and you may be done for). Also, getting to Jeuno is risky at the level because there are some highly aggressive and nasty mobs on the way. Riding a Chocobo is highly recommended.

Ultimately, this is only a mediocre get rich quick scheme, and one which doesn't grow with your skill levels. You will always be limitted by both how much you can hold and how much the grasses sell for at the Jeuno AH. Still, it is quick and easy... But if you want something that earns it a bit more, then you should keep reading...