Clean out the fireplace with the broom and dustpan, removing debris, old wood and ashes and soot. Vacuum to remove remaining dust, and get rid of the bag immediately.
Decide if the fireplace will remain open and simply unused, or if you want to brick over the entrance. Choose to leave the fireplace open for the sake of ambiance, as long as it is safe to do so.
Repair or replace any broken brick or mortar before painting.
Paint all the brick with an oil-based primer to keep soot from bleeding through on to the paint. Use a flat finish that will hold additional layers of paint. Include the inside of the fireplace since it won't be in use. This provides a more finished final appearance. Make sure you work in a well-ventilated room.
Paint the brick white or another color that complements your room, using the roller and paintbrush where necessary. It will need several coats of paint, because brick is porous.
Finish with a final coat of the sealer to protect the paint.