Do you mean farming, or gardening?
Farming is just killing stuff for the drops, gardening is where you (and your Moogle) grow stuff in your Mog House.
If you mean farming, I suggest checking out what mobs you can handle at your lvl vs. what mobs drop good stuff. Crystals are good, esp. Fire and Wind. Beehive Chips from bees are good, and Silk Threads from Crawlers are good. You will get a better feel for the good farming items as you get more familiar with the game.
If you mean Gardening, check out this link:
http://www.swapsonian.com/pikko/index.html
If you are looking for some gil, there is also Fishing. You also might want to check out H.E.L.M. That stands for Harvesting, Excavating, Logging, and Mining. Here is a handy link for H.E.L.M. items and maps:
http://www.hi-ho.ne.jp/gulif/ziq_topeng.html
Hope this helps.