Joel, Recheck battery to make sure it has good charge >12 v and all battery and starter wiring,plugs,connections are clean,tight and undamaged. If you remove spark plug and engine spins freely from the key, the engine compression release or valve clearance needs adjusted. It is probable the solenoid or starter is faulty, the starter Bendix/drive may be sticking. If you can, give the starter a couple sharp raps with a hammer or bar and then try to start.
Check the attached links,instruction and guides to help diagnose starting issue. Good luck
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http://troybilt.custhelp.com/app/answers/detail/a_id/86/~/clicking-sound-while-attempting-to-key-start---riding-mower
What Are the Symptoms of Bad Starter Motor on Riding Mower Engine
My Bolens lawn tractor won start It just clicks when turn the key even...
Craftsman riding mower won start Just clicks the starter motor

Either you have a bad starter solenoid or (more likely) the plastic gear on the starter shaft (it engages with the flywheel to crank the engine when you turn the key) has stripped out. Remove the starter and take it to a small-engine shop; they'll have the part.