First question - what is meant by 'won't start'? Does this mean there is no sound, and no crank when you turn the key in the ignition?
If nothing happens when you turn the ignition key to the "Start" position, it means that the starter motor doesn't turn over the engine.
Most commonly this could be caused by a low/dead battery, or there is a poor connection between the battery and starter, or there may be a corroded / loose connection in the battery-starter heavy gauge cables.
Check the battery and the terminal cable connections. If they look really corroded, you need to clean the battery posts and cable connectors or replace the cables and try to start the engine. Also check the starter. The solenoid attached to the top of the starter can fail, as well as the components inside.
Thoroughly test and diagnose, before replacing any component. Test and replace the starter solenoid, starter contacts, starter assembly or repair starter circuit as required.
If engine cranks over but won't start:
There are many possible causes, but we know it is neither a battery nor a starter problem. Just remember: a vehicle will always require air, fuel, and the ignition (spark) to operate.
Check the fuel pressure and flow - using a loaner gauge from any local auto parts store,. You may be out of fuel, or the fuel isn't getting to your engine. If the wrong amount of fuel is being injected into the combustion chamber, or being injected at the wrong time, the engine will not start. Again, this will be a case where the engine cranks over, but will not actually run.
This could be caused by a few different parts including the fuel pump, fuel filter, fuel injectors, and fuel lines. Some of those options are a stretch, but do have an effect on the system. Start with the fuel injectors, and make sure they are clean.
If it's not a fuel problem, the electrical spark isn't getting through to the spark plugs. Check the spark. Without spark to ignite the fuel mix in the combustion chamber, the engine will not start. It will turn over or "crank over" but will never actually run.
Spark issues are not limited to the plugs. Check the spark plug wires, the distributor or module, and plug gap for the correct setting (distance from the center-to-side electrode),. These all have a major effect on the function of the primary and secondary ignition system.