First you need to eliminate possibility of a bad starter or solenoid:
1) have an assistant turn ignition key and hold key in the start position, then...
2) lightly tap the starter solenoid with a heavy object or tool; or
3) wack the starter motor body.
If no response, test for voltage at the starter solenoid when key is in the Start position.
If no voltage present with key is in start position, then you most likely have a worn "ignition switch". This electric switch is mounted in (or behind) the ignition barrel lock, in the steering column.
If you have access to a wiring diagram, you will be able to disconnect the harness plug and test for continuity at the plug, when key is moved in each position.