Wash it and fix any defects. I would then think about using a solid color stain like Sherwin Williams Woodscapes.
Otherwise, use something like a high bond primer and glaze coats of nice acrylic finish mixed into an acrylic medium like Floetrol. Do not over thin or use water or you will destroy the properties of the paint.
Use a thick nap---1-1.5"---roller cover chase with a brush.
An alternative would be to stain or paint it to a solid white and then wipe away finish to give it a whitewashed look.
You can get solid color stain and paint of course tinted to any color. Think about something other than WHITE WHITE for the fireplace. I think white is too stark for most interiors and can make everything around it look dull and dirty instantly.