I would first decide if you really want to do this. AFter all this is forever. it is almost impossible to go back. Have you thought of resale and how appealing it will be for another person and if it could hold your sale back.
I have to say this because its very true. I have my fireplace painted and I paint around 5 or 6 each year. In most cases the person wants to lighten up the room, make it bigger and do away with possibly an unsightly brick and most of all to reduce the dominance of the fire place within the room.
What I do first is to thoroughly clean the brick of soot, grease, and dirt. Just good ole soap and water will work and on bad areas use a strong cleaning agent, then remove residue with fresh water.
Secondly, repair any cracks or breaks or defects with a good paintable caulk.
Thirdly, Decide upon your color and always go a semi gloss or shiner. The more the gloss the easier it is to clean and the brighter it is. Mine is Pure White in semi gloss.
Fourth, apply several coats of a good latex primer. Let each one dry before you add another coat. apply liberally on the mortar to fill it in.
Fifth, apply about 3 or 4 coats of the finish paint. You will see that the more the coats the nicer the finish and texture of the job. I put about 7 coats in all on mine then each Spring I re paint it from the wear and tear during wood burning season.
Use a good "paint dealer" paint, like Sherwin, Ben Moore, or Kelly Moore.