Starter spins, but doesn't engage the flywheel? Are there any grinding noises, or just the starter spinning? If no grinding, just the starter spinning, then it's probably the Bendix drive on the starter isn't sliding to engage the flywheel. Other option...the teeth on the flywheel are chipped and broken, so they can't engage with drive.
Probably have a broken bendix spring. There are two long bolts that go the length of the starter housing and thread into the bell housing - unscrew them (but don't remove them) and the starter will come out. Watch that you keep the starter housing ends in place, cause with the bolts loose it'll come apart. A couple of coarse threaded nuts on the bolts will hold it together while you work on the bendix.
The bendix spring is held on with 2 bolts with lock-tab washers. Make sure you reinstall them correctly with the new spring. Fairly simple repair, but make sure you retorque the spring bolts - and a little loc-tite is recommended. I've had the bolts back off and fall into the flywheel housing (twice). Good luck.