Fireplace renovation cost

I have a 1920’s Spanish revival house with a fireplace. We don’t know the last time it had any maintenance but I’m guessing never. When we had a fire it turned the fireplace wall black on the inside. I’d love to get it fixed but don’t know where to start or how much it would even cost.
I’m in Southern California. Can someone recommend any tips for me? My husband is pretty handy but I think this is well outside anything he could try to do on his own.