Thank you for not painting the brick, Karolyn. This will prove good for your soul as someone in the future will not put a curse on you for painting it in the first place.
As for cleaning, vacuum the brick and then wash and rinse. The best choices for washing are either Simple Green if the fireplace hasn't been used much or Tri-Sodium Phosphate (TSP) if it has. Be sure to wear gloves and eye protection if you use the TSP and that is available from home centers and paint stores.
Since you want to change the look of your fireplace, why not consider adding a mantle or surround. This can be done rather easily but it can get expensive, depending on the extent of the detail and moldings you choose. I believe that there are kits available and you might do a computer search just to get some ideas. Installation can be either a DIY project or best left for a pro, depending on the complexity of the surround you choose and your level of expertise. Something to consider anyway.