MY 1999 Pontiac Sunfire is overheating. I changed the thermostat and put coolant in it.The engine gets hot but the radiator stays cold.

u should have used a radiator flush before pouring the coolant your water jackets must be choked with rust they r small hole which r blocked up by rust or corrosion

Could it also be the water pump is not working

if u used water as coolant more then proper coolant it could be the water pump as well

and check for mud or dust also , it should be seen after removing the radiator cap floating above the water surface if your water is brown coloured its rust on water pump and in the lines

also check your sparkplugs and ignition timing if distributor is present

is thermostat in backwards? if corrosion is apparent, replace all hoses, radiator could be clogged, and replace water pump , sounds like everything needs to be replaced, cheaper to do it all at once instead of one thing at a time replace radiator as well