Rhinoman has the good advice assuming the key is not broken off in the lock...
often times on older vehicles the ignition switch wears to the point where you have to apply slight turning pressure to the steering wheel or to the shifter lever (if it is automatic) in order to get the key to move.
This feature is in place on automatic transmission vehicles to provide the same fuction as the clutch ignition switch does in order to prevent the operator from trying to start the vehicle while it is in gear.
Hope that this helps!