I find making money isn't that difficult in-game. The important things to keep in mind are how to increase your profit, and decrease your costs.

First and foremost, take a look at what's selling. This can be the most daunting part to start with, although you can get a little hang of it by checking the AH for every item you snag while xping. If it sells multiple times a day, that's a good item to sell (Barring if the item is rather difficult to obtain, like brown casket equipment). If it sells probably once a month, you're likely better tossing it at an NPC. Something can be selling for 50k a pop, but if people aren't buying you're better off using that slot for 1k items that fly off the shelves. Moving your product is one of the most important aspects of making money on the AH.

On a related note, look to alternative ways to make money than the AH so that you can still make money when your seven slots are full. One common example is to check NPC selling prices. Many times, you can sell to an NPC for close to what you can sell it on the AH for... a small loss may make up for being able to use that AH slot for something else. Rarely, you can sell it to an NPC for more than the AH. Quests are another big example. On my server at least, both black tiger fangs and bat wings can earn you MORE money turning them in for the quest than selling them on the AH, so you can make a profit AND work on raising your fame for higher NPC selling prices and more quests.

Soloing is very good for getting money early on, and nowadays isn't that difficult. In addition to the free money from Grounds of Valor, you should use your prior research of what does and doesn't sell, and use that to decide where to go. Good targets are bats and tigers (for the aforementioned reasons, tigers moreso as the hides can sell on the AH), sheep (turn the skins into leather and sell stacks of it. Bring windurstian tea leaves, distilled water, and dark crystals so you can craft on the go), bees (Beehive chips, either sell as is or make honey or beeswax.), and beastmen (Straight up gil drops, beastcoins, and many of their unique drops are often bought for desynthesis). There are plenty of other things with decent drops as well, so keep an eye out.

Another possible money maker, although you may wish to get some gobbiebag upgrades first, is HELM (Harvesting, Excavating, Logging, Mining). If the place you're gonna solo at has points for these, go ahead and give it a shot. Just pull out your tool whenever you come by a spot and use it up. This works best if you have the carpentry/smithing skills to convert ores and logs into stackable lumber and ingots. On that note, fishing can be a decent money maker, although that's a little harder to justify doing in the middle of soloing, as you can fish a single spot until you hit fatigue for the day. Which brings me up to another point...

Check out daily/hourly/weekly/whatever activities. There's often a reason why certain repeatable quests and events have limits on how often they can be done. Things like fishing, chocobo racing, chocobo digging, gardening, repeatable quests, add-on scenario keys... anything that has a limit on how often you can do it over time is often good for money. Essentially, like most content it is balanced around getting a certain amount of stuff over a given period of time. Unlike killing mobs, which places that limit with a drop rate, time restrictions keep the same "loot over time" balance while requiring you to only spend a fraction of your time on that activity.

Finally, all this advice is primarily for lower level stuff, and I haven't even touched on all of that. Keep an eye out for anything that looks profitable and take a chance. Competition will be your biggest issue in making money, both in gathering the materials and on the AH. There are many hidden nuggets out there that nobody bothers to work with, but which can earn you a decent chunk of change. Especially if you're the sole provider.

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So, the secondary part. Not spending your money. First and foremost, consider how badly you need something. For example, if you're soloing the level differential will be in your favor, and you don't really need to spend several hundreds of thousands of gil on a massive +acc piece. Secondly, compare AH and vendor prices. You'd be surprised sometimes which one is the cheaper option. Third, keep small purchases in check. Things like chocobo rentals, outpost teleports, they can add up over time (Though, on that note, spending 10 minutes to walk someplace to save 500 gil is likely not worth it, as you can easily make that 500 gil in the 10 minutes saved. This is mostly a concern for shorter distances)