Bad thermostat? check to see if your upper radiator hose is getting hot and that coolant is flowing into the radiator.

If you want to see if the stat opens you need to start with a cold engine. You can remove the radiator cap when the engine is cold without risk of getting burned by escaping steam. Then you start the engine and wait for the coolant to start flowing when the stat opens. If the car heats up and the stat doesnt open then you should replace it.

You said the fan comes on...which usually means the radiator is getting hot enough.....perhaps your radiator is getting plugged internally or you have a poorly functioning water pump?

Check these things out and let me know how things are.

Good luck and be careful.