Put your hand on the starter solenoid and try to turn the engine on, you should feel it click and this indicates that the solenoid is working. Just to verify, hook a meter up to it and put one lead on the positive wire GOING TO THE STARTER so you can make sure the voltage is getting through.
If nothing happens when you hit the switch, you can turn your ignition switch on and bypass the switch by getting a small piece of wire and put one end on the POSITIVE BATTERY CABLE COMING FROM THE BATTERY that's connected to the starter solenoid and touch the positive wire COMING OUT OF THE IGNITION SWITCH INTO THE SOLENOID (usually a very small red wire connected to it). This will activate the starter solenoid and cause the starter to work and your engine to run. If the above works, your ignition switch is bad replace it and if the solenoid doesn't trip, then it's bad.
If your solenoid is tripping and the voltage is getting through, try tapping the starter lightly with a hammer to see if this causes it to work. If by any chance it does work, replace your starter because it's about to go.
Make sure your battery is good before doing any of this, I've seen batteries with 12.5 volts and no amperage. Also make sure all cables and connectors are tight.
In short, if the voltage is making it all the way to your starter, and the batteries good, the odds are the starters bad or your engines locked up.
Let me know how it goes.