It takes 4 things for an internal combustion engine to run:
Compression, Spark, Fuel, and Air.
Eliminate them one by one, and you'll make this much easier.
For example, is your air filter clogged? No, okay, then you just eliminated 25% of the possible problem.
Does it have compression? Yes? Great, you've now cut the pie in half.
Is it getting fuel (are the spark plugs wet)? Yes? Okay, than it's an ignition problem.
Check to see if you have power to the coil. You do? Great. Now see if you have power out of the coil. You don't? Replace the coil.
Fill in the specifics accordingly, and you'll find your problem.
Take care,
ÅChris