To unlock Chocobos and get the "Chocobo Jockey" achievement, you must first reach story Chapter 3. Then, talk to Wiz at the Chocobo post (Duscae map). When arriving in Chapter 3, the "Friends Of A Feather" side quest will automatically be marked there. Accept his "A Behemoth Undertaking" hunting quest and kill the Behemoth (marked as mission target). After killing the Behemoth, return to Wiz and talk to him again. You can now interact with the Rent-A-Bird machines that are found at most outposts and parking spots. It costs 50 Gil per day to rent a Chocobo. You can press RT to whistle for your Chocobo wherever you are. After the rental period has expired, the Chocobo will run back to its outpost and you must rent a new one. You can also rent one for several days at a time.