Tradeskills take the most time out of any of choices among varous way of making ffxiv gil, but the payoff can be well worth it. While you can choose any profession you want, here is what I recommend on how you should approach tradeskills:
Choose one gathering and two crafting professions. That will allow you to maximize your profits. Ensure the gathering profession provides materials for each crafting profession.
Don’t try to level them all the way up immediately. Every time I would gain five levels in a job, I would switch. This allowed me not to get bored, and to sell off the items I made.
Don’t be afraid to spend a lot of gil buying materials. Some materials can only be found by item drops from monsters, or from other professions. Once I hit level 25, I spent 10,000 gil buying materials to craft the items for my trade profession.
Do some levequests. Levequests aren’t necessary for gathering, but for crafting it can be. This will allow you to earn some gil and experience, while not using as many materials just making items. I recommend not turning in high quality items though, just normal items. Grand company quests can be run as well to earn experience and seals, however they won’t earn any gil.
Try to make high quality items. High quality items always sell, no matter what level you are. So strive to make high quality items. You will make your money back, and then some.