Diagnosing starter problems isn't too hard. Starter problems can come from a variety of sources, like a dead battery, a bad solenoid or a malfunctioning ignition switch. If you turn the key of your car and nothing happens, you can easily troubleshoot the problem by following these steps:
Check the battery by turning on your headlights and evaluating their brightness.
If you have a voltmeter, use it on the battery. A completely charged battery will have about a 12.5 voltage reading and a dead one will be less than 11.8v. If your battery is dead, charge it before moving on to the next step.
Test the ignition switch to see if it's sending power to the solenoid by seeing if the lights on the display panel dim when the key is turned. If they do, the ignition is working properly.
Double check the ignition by getting underneath the car and disconnecting the ignition switch cable from the solenoid. Check the voltage at this wire and make sure it reads 12v. If it doesn't, your car may have a broken ignition switch or wire connection between the battery and ignition switch or starter and ignition switch.
Next, test to see if the problem is the starter motor itself. You will do this by connecting the battery to the starter motor without using the solenoid. Turn the parking break on and put the car in neutral. Then, using insulated pliers or a screwdriver, short circuit across the two electrical connectors. There will be sparks, but it is important that you keep the tool there until there is a good connection. When there's a connection, the starter motor will spin but the engine will not be cranked. You should hear a normal motor sound. If the motor makes strange noises or doesn't spin, it will need to be rebuilt.
Finally, check to see if the solenoid is functioning correctly. Again, make sure the parking break is on and the car is in neutral. Look at the two connectors you just worked with and note that there is a short, heavy wire connected to the one on the right hand side. Measure the voltage of this wire while grounding the negative lead. Have a friend turn the key and watch the voltmeter. If it doesn't have a reading of 12v, the solenoid isn't sending power to the starter motor. If this is the case, the solenoid will need to be rebuilt.