HI Laurasop, you can use whichever tile you like most, apply tile adhesive to the back of the tile and some to the wall using a small notched trowel ( adding some to the tile helps if the wall is not completely flat and straight) you do not want this gobbed on, a little works, enough to cover the tile but not too much that the tile slides down the wall
Constantly check with your hand to make sure that the tile is flush and even where all 4 tiles come together and that your lines are all lining up really well.
Remember that any slight heave or dip in the wall will reflect in your "grout line" a heave will open it and a dip will close it, you might not notice it until a few rows have been layed.
Start your full tiles from the outside of each side and work your way in both ways so that your cut tiles will be in the center back (for a corner fireplace), and start a full tile dead center for a flat wall center of the room fireplace use spacer to keep everything even and straight.
Wait until it is all really dry to grout, I like to wait 36-48 hours to be sure
ANy other questions just ask , good luck