Cut a piece of plywood to 10" wide by 4' long. It's easiest to have the store cut this for you.
Cut out 1.5"x1.5" squares from the two corners that will rest agains the wall beams.
Place the shelf and secure it with 1.5" wood screws.
Depending on your preference, you can start placing tile now or wait until you have all the beams in place.
Cut the tiles to fit on the shelf. You can usually use scissors or a cutting blade.
You may be able to cut the tile in place using a cutting blade. Simply score the tile (make a small cut along the line you want to cut out), and fold the tile and it should snap neatly.
Here is the completed shelf.