If you are referring to using chocobo mounts, you first need to use the item given to you, I believe it was called "Chocobo Whistle", or something similar. Once that is done, you will be able to use mounts. If you refer to summoning your chocobo in battle, you need to use gysahl greens (buyable from various vendors in areas such as Bentbranch Meadows, the Gold Saucer chocobo square, etc) to summon it.