My heat doesn't work, A/C blows lukewarm air

I live in cold Michigan. My heater doesn't work, brrrr!! My temp gauge will stay at normal after warming up for a while, but after I turn on the heater the engine temp gauge drops to cold. In the summer, the A/C blows cool, not cold air like it use to.
If it is my heater core, how to replace it (pics please if available).

Thanks in advance

mine blows hot air...i think my a/c amp went out on me...and it's hotter than hell here in mississippi