Sell items at the Auction House. When you are first starting out, this is the easiest and most reliable way to earn cash. Try to collect items in groups of 12, and sell them as sets. By selling in bulk, you drastically increase the value of each individual item. Crystals are always in high demand, and you can collect them at virtually any level. Cast signet on yourself and start killing creatures. You will pile up a ton of them in a short amount of time, and all it takes is a quick trip to the AH to make some fast cash. Similarly, any enriched, enchanted, or rare item will also go for big bucks, especially if it happens to be the only one on the market at the time.
Fish. This is also an easy way to make cash, albeit time consuming. Go to the AH or a street vender to buy a rod and bait. From there, it’s as easy as finding some water to cast your line into, and then reeling in you catch. It takes a little bit of practice at first, but once you get the hang of it and your character’s skill improves, it’s a very profitable enterprise.
Pick up a craft. Synthesizing items, woodworking, blacksmithing, and alchemy are all examples of crafts and skills in FFXI that pay very well once your character has the skills required. Go to a guild to get yourself started, and the patrons can fill you in.
Go questing. Although this has a relatively minor cash incentive, most missions and quests will at least give you some form of compensation, be it cash in hand, or a rare and valuable item that can be hawked at the AH to another player.
Sell unused or unwanted items out of your inventory. You can turn your inventory into a permanent shop by simply placing items in your “Bazaar”. Players can shop and buy items from you by simply using the “Examine” command. It doesn’t always pay well, but every now and then you will sell something at higher than market value and make a bundle.