Irana, and others, there is a very simple yet powerful method of acquiring wealth.
Specialization
... is the most important thing. For crafting, gathering, or battle... specialization is what separates the exceptional from the mediocre. You will never make more gil by levelling every class than you would have by simply raising a single craft to max and producing items at the maximum level. All the time and gil spent on raising different classes comes at the cost of gil you could have made from spending time on your best class. So, don't think that because you level every class to 50 you will have some advantage.
That said, for starting out... gathering is the best option. You can pick from mining, botany, and fishing. By doing the leves, you should have no problem figuring out how and where to gather.... and if you check the market wards after a gathering session, you will soon figure out what people want most. Meeting demand and providing items that people want is the role of a producer. Sometimes the hardest to get items are not the most valuable. Of the three gathering classes, you may choose whichever you prefer most, or you can try and figure out which class makes the most gil... but these conditions change often so it is far better to spend time on the class you find more enjoyable.
Now, because it is far better for everyone to specialize... if you are a Blacksmith.. do not bother farming ore for yourself. You are still paying for the ore in the form of time.... and time is money. If you spend time farming ore, when you could be smithing, you are losing gil. If you have the choice of either farming ore, or smithing, spend 100% of the time on the class that makes the most gil in the least amount of time. Buy all the materials you need from others.
A basic plan to follow, if these concepts are hard to grasp is the following: Level a single gathering class, sell what you gather and you will learn by trading through the market wards what is best. You should be able to decide what to do from this point on, but save your gil ^_^, saving is important.