You might need someone to bang on the starter while you try cranking it over..

Its pretty common, well that ive experienced, not sure if thats your problem, might be worth a try.

I had one of those didn't need a helper though, a couple whacks and it would start. If it is that a new starter will cure it. Also could be the ignition switch as others have suggested too. Its a common issue as the switch gets older. The typical fix would be to add a relay into the ignition switch from the battery using the switch wire as a trigger/switch for the relay.