Despite the undreds of things that can go wrong, the three main things that can cause an engine not to start are 1. Lack of spark2. Lack of compression3. Lack of fuelSince it starts sometimes but not others, i'm willing to bet anything that it's one of those three, probably lack of spark or lack of fuel. If you can see where the spark plugs are connected on your car [mine has little wires right on the front labeled 1-6, 1 for each cylinder] unplug them then push them back on. That could solve your problem, one of them may be loose.If its fuel, it could be your fuel pump. Turn the key on and have someone listen to it to see if it turns on too. The fuel pump is typically located right beside the fuel pump, which should be easy to find.I would seriously doubt that it's lack of compression, since the car runs sometimes.