So I just downloaded Logic Pro and I don't know if I'm just stupid or something, but I just can't figure out how to define what the inputs and outputs are... Any help?

Apple has some tutorials on their site on how to use. Do a google search and you will come up with a bunch more as well.
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Quote by BrianApocalypse
Good call

Man, you should be a mod, you know everything.

Logic is not a beginner's program since it's very open (it's basically meant to emulate the routing possibilities in an actual, physical studio while adding more on top of that). The best way to get started is just to use one of the template/example sessions that should come with the program. That will give you a basic idea of how things are generally set up. Then check out the routes that thrice_removed suggested.