First thing is ALWAYS have signet on when you fight mobs. Without signet you won't get any crystal drops. Crystals are key.
Then next question is whether to just sell your crystals (in stacks of 12) or to craft with them. Yes, crafting can be very lucrative - if you do it right.
Instead of selling all your fire crystals, check your Auction House for how the supply of copper ingots looks. How many stacks sell in a day? (You'll find copper ingots under Materials -> Smithing; click on one entry and then sort - you'll find in alphabet order after that.) Go to San d'Oria, buy copper ore from the Smithing Guilding - make a stack of copper ingots, put them on the AH for 999 gil less than the previous sale (minus one thousand, plus one). Just realize that putting up more stacks than sell, won't make them sell. Don't flood your markets. To make a copper ingot "use" a fire crystal, that will open a grid that you can place 4 copper ore into, and then click OK. You'll see an animation of you crafting, followed by either a pop showing the synth was successful, or a break, where you lose the crystal and possibly some of your ingredients. Don't fret breaks, they happen. All the synths I'm giving you are in the range of beginners.
Instead of selling all your dark crystals, check your AH for how the supply of sheep leather looks. (Sheep leather can be found Materials -> leathercraft.) How many stacks sell in a day? Since you're in San d'Oria, go to the leather (Tanner's) guild and buy sheepskins and distilled water. See if you can buy a stack of windust tea leaves on the AH. The synth is "use" dark crystal, place sheepskin + windurst tea leaf + distilled water; you'll get the animation and pop a sheep leather is synthesized.
You can also craft all the ingredients for stone arrows. See how many stacks of stone arrows sell per day. Buy the arrowwood lumber from the Woodcrafter's Guild; make stone arrowheads by using a wind crystal and 2 flint stones. You'll also need chocobo fletching, which you can craft with wind crystal and 2 chocobo feathers which can be purchased from an NPC near the chocobo stable in Southern San d'Oria. Since you can obviously craft chocobo fletchings, you can also craft a stack of those and place them on the AH.
Between selling your crystals and these 3 beginner synths, you should be able to get all the gil you need.