To locate them, look at each caliper (the metal housing that holds the brake pads). The bleeder screw is usually a small, slightly recessed valve located on the top or side of the caliper. It's typically a slightly smaller diameter than the caliper bolts and may have a hexagonal head for a wrench. You'll need a wrench (usually 8mm or 10mm, but check your truck to be sure) to open it when bleeding the brakes.
Important Note: Bleeding brakes is a somewhat involved process and requires specific steps. If you're not comfortable performing this task yourself, it's best to have a qualified mechanic do it. Improper bleeding can lead to a dangerous loss of braking power.