If they've not been off for a while , especially if they're alloys, they'll be stuck to the hub.
A few good clouts on the inside of the tyre (NOT the wheel) should see it come away eventually.
Once it's off clean off any corrosion with a wire brush and put a thin coating of copper grease on the cleaned surface.
Support the car by placing something solid like the spare wheel or a large block of wood under the sill while it's jacked up and lastly, watch out for rebound of the hammer off the tyre.