First, welcome to OCC...............
First, make sure the PTO switch is in the OFF position. The Brake Pedal has to be depressed. Both of these are needed, to roll the engine over.
The CH18 has a Magneto Ignition System. Thus, if you have no spark, it can mean one of two things. (A) The Ignition Coil is bad, or (B) the "Kill" wire going to the Ignition Coil is grounded (shorted).
First simply test would be to remove the "yellow" wire from the "M" on your ignition switch. See if that bears any results.