This "job" will be available in the very beginning just after you've helped out Dan Hagar with his task for you. (The one where you have to wipe out the Bandits) Just walk out of his house, and in the back there will be a cave, enter it and enjoy whatever awaits you! (no spoilers!)
Worried that you'll miss these jobs during your playthrough? Don't! You will be clearly informed by a pop-up on your screen whenever one becomes available. (They will become available as you progress through the campaign).