2002 ford explorer, bought starter off of rockauto.com, hooked it up and it just makes a spinning sound?

I checked all connections and still nothing. Where do I start to diagnose my issue? What could it be if everything is hooked up properly?

Spray some wd40 on the starter spindle. That spindle is supposed to slide up and hook up with the teeth that will turn the engine.

Defective starter. Was this the original problem. Might have bad teeth on the flex plate and starter won't engage the flex plate

Remove the starter and get it bench tested at a shop or parts store if they are capable. Return if defective.

Hi so what do you expect from a 14 year old truck is the question MONEY MONEY needs to be spent getting it fixed or getting down and dirty with it and getting an education about car mechanics.

If all connections are correct and you reinstalled any shims that fell out when you removed the old starter…
Then you have a bad solenoid. It needs to pull in to allow the gear on the starter to slide forward and engage the flywheel. Currently it is not doing that.
As suggested, remove it and have it bench tested locally.
If there were any metal spacers (shims) that you didn't reinstall, do so first. They set the starter back so that nothing binds and prevents the gears from engaging. If you have ever heard a car make a screeching or high pitched screaming sound while the starter was spinning, that was the reason for it.

But Do Double Check that you didn't leave one small wire off the starter solenoid.