Turn on the headlights and check their brightness. If they are dim; have the battery recharged and load tested. The load test measures the amount of energy in the battery. A voltmeter cannot be used to test the power remaining in a battery. Replace the battery if it fails the load test.
Once the headlights are at full brightness; continue the test by turning the key to the "start" position which watching the headlights. If the headlights dim way down or go out when trying to start the motor; disconnect the battery cables and thoroughly clean the inside surface of the cable clamps and the outside of the battery posts with a tapered reamer type battery cable service tool. A round file can also be used; but the reamer does a much nicer job. A properly cleaned cable clamp or battery post should have a shiny ring around it. Reinstall the battery cables, and tighten the clamp nuts until they cannot be moved by hand pressure. Then re-try the headlight test.
If the headlights are still dim or go out in this test; have the battery recharged and load tested. Replace the battery if it fails the load test. If the battery passes the load test; but the headlights still dim or go out when trying to start; this means that either the starter is defective; or the engine is jammed internally or frozen up.
If the headlights stay bright during the test; but the starter does not run, the most likely cause would be a bad clutch pedal position switch (only used on manual transmission cars), or a bad neutral safety switch (only used on automatic transmission cars). A defective starter relay or a defective starter solenoid would also cause this. And a defective ignition switch could cause it; although it is not as common as the preceding parts.
You can bypass the clutch switch, starter relay and the neutral safety switch in one operation by disconnecting the small diameter wire from the ignition switch at the starter solenoid (this wire is the only small wire that goes by itself to a terminal on the starter solenoid), and briefly touching a jumper wire from the battery positive cable to the terminal where the ignition switch wire was removed. Make sure the transmission is in Park or Neutral before attempting this; and keep hands, body parts, tools, and loose objects away from the fan and fan belts. The starter should crank the engine when you touch the jumper wire to the starter solenoid terminal. If it does; this means the starting problem is caused by the clutch switch, starter relay, ignition switch, or the neutral safety switch.
If the engine does not crank when the jumper wire contacts the solenoid terminal; the solenoid or starter is defective; or the engine is frozen up.