For a modest cost, tile will alter the look of a fireplace and dramatically improve the overall character of a room. The major difference in tiling a fireplace compared to other surfaces is the type of mortar required. Use heat-resistant epoxy (up to 400 degrees F) when tiling over wood; use heat-resistant cement-based mortars on masonry surfaces.
If a brick surround and hearth are stable and in good repair, you can tile over them. Clean them thoroughly, removing any soot, which interferes with adhesive; flatten any high spots with a rubbing stone. Spread a thin coat of heat-resistant mortar over the brick surface, leveling the mortar with a 2x4 set against 1/2-inch screed boards tacked into the mortar joints on either side of the opening. Let the skim coat dry and use it as a base for the tiles.
If the surface is in poor repair or you don't want to apply a skim coat, cover it with backerboard. Backerboard must be used when applying tile to a metal surround because most metal surrounds are not strong enough to support the weight of tile.
If you plan to tile the surround, expect to spend four to six hours tiling. This does not including prep time. For the hearth, plan to spend about three hours tiling. Clean the fireplace and level the surface before you begin.