Technique Number 1: Farming.
Farming is the first way that everyone makes money...
You pick up the monster drops and sell them to shops, or other players via personal shop.
To use your personal shop, click on yourself, then at the top use the drop down menu. Select 'Personal Shop', then stock up and sell. ^_^.

Some drops will be extremely expensive. Cores, Epaulettes, Double Slot items etc.. Can be sold in the personal shop for a huge amount more than the NPC shops will buy them for. So don't make any mistakes about pricing items.
Technique Number 2: BLSH (Buy low, sell high)
Most merchants' favourite way of making money.
You buy an item at a fairly low price, and then re-sell it for a slightly higher price than that which you paid. Repeat however many times you can be bothered.
Remember: Higher level items and lower level items take a lot longer to sell. Mid-level items are best for this technique e.g. Upgrade Core Mediums, Force Core Highs, Bluestin/Titanium Armour, EoX+3/4, Level 40-100 Jewellery.

Last but not least:
Technique Number 3: Crafting
It's a pain. Yes. I know, but stick with it. It'll make you millions in the long run.
First off, once you reach level 50 you'll get the Map Code to Port Lux. Once there, the Core Alchemist will have a quest for you, the reward for such will be a Transmuter of your choosing: Armourset, Martialset, Battleset, Artifact etc..
Once you've picked your type, you can start breaking down armour, jewelery and weapons into their crafting materials.
After you make your items, you can sell them at any price you want. Personally I make them as cheap as possible, so I make a tiny bit more Alz than I spent, to increase sale speed.
Adept Items sell the best, as more people favour Skill Exp. So once you're able to make Adept in that certain type, it's advisable to stay with that until you can do the next type. Although a lot of people do all the enchantments anyway.
Make sure you're always keeping prices around the average or you'll never sell anything.
If the prices are too low you'll know about it when your items disappear extremely fast. If so, raise the price a bit next time.