"Won't start" is pretty vague. It could be hundreds of posibilities.
Dead battery, Low battery, bad ground wire to engine, bad power wire to starter, bad power wire to battery, bad connection at battery post or terminals, blown fuse, bad key switch, bad starter, bad fuel pump, bad injectors, out of gas, neutral safety switch, seized engine, broken connecting rod, no compression, no spark, bad oil pressure switch, broken timing belt, plugged fuel filter, bad starter solenoid, broken rotor, cracked distributor cap, Anti-theft device, water in/on engine, vacuum leak, bad ignition coil, blown computer, faulty cam or crank sensor ...
When you turn the key to START it what happens?
1) Nothing happens? (dead battery?, loose battery cables, or it's not in "park" all the way.)
2) You hear a "click", but nothing else? (loose battery cables, bad starter, bad starter solenoid, or broken engine)
3) You hear the starter turning, but the engine doesn't turn? (low battery voltage, bad starter or bad ring gear on flywheel/flex plate)
4) The starter turns the engine, but the engine won't start up? (out of gas, bad fuel pump, plugged fuel filter, broken timing belt, no spark)
5) The engine turns over, but very slowly. (weak battery, worn starter, out of engine oil)
6) the engine turns, but there's an awful banging noise? (something is broken inside the engine)
What happened last time you drove it?
1) Drove home fine last night, just won't start today?
2) Was driving fine, and it just died on you?
3) You haven't driven it in a while. It's been parked in storage?
ADDED:
Push the clutch pedal ALL the way down.