Narrow it down a little at a time. Start with fuel. See if you can use a can of starting fluid to get it to run briefly. If you can, then it's a fuel delivery problem. Check for a blocked fuel filter or worn out fuel pump. If you get NOTHING from the starting fluid, pull a plug and see if it's wet. If the plug is wet the engine is flooding.
Lay the plug on a solid metal surface, reconnect the plug to the wire then have someone try to start the engine for you while you watch the plug. You should get a nice strong spark. If not, you have an ignition problem.
Last, run a compression test (make sure you mark all of the plug wires so that they'll go back to the same place) If you have low compression the engine will have difficulty starting.
If spark, fuel and compression are all functioning properly the engine will run.