Depending upon year of Ford, It could be either the starter solenoid on the fender, pos cable hooked to it, or, the ignition switch bad in start position.
If a newer model, either a bad starter relay, or starter motor.
Or for both cases, the neutral safety swich misadjusted or broken, if A/T. If manual trans, clutch safety switch malfunction.
But alas, if the truck is of the solenoid style, one can always take a pair of pliers and short across the solenoid to engage starter to start.