I have an '02 Pontiac Montana and it is over heating and blows out cold air. Yes the fluids are good and we just replaced the thermostat and it still is over heating and not blowing out heat.

You have either an air pocket in the cooling system for the engine, a bad sending unit for the temp gauge/dash warning light, a blown intake gasket or blown head gasket.

I have a 2000 montana the heat don't work it does when is park but when I drive it for 15 min it blown cold air what it can be

A bad thermostat, a bad blend door motor, a broken blend door, plugged heater core. Check and see if you're low on anti-freeze, if it is low fill to proper levels in radiator and overflow bucket, then look for a leak(intake gaskets are EXTREMELY common on the 3.4 liter) If its not low let the van warm up, carefully grab the upper and lower radiator hoses. If they are both very warm/hot then move on the the heater hoses and do the same. If the radiator hoses are both not hot then its probably a thermostat. If the heater hoses aren't both hot then its most likely a heater core. If all of the hoses are hot then its probably a blend door, or blend door motor.