How Do I Lay Out 18 X 18 Inch Tile In A 12 Ft X 12 Ft Room?

Find the center of the room in both directions.  Determine what size tile you will end up with at the edges (you want to avoid small pieces anywhere). If starting in the center will produce a small piece at the edge, shift the centerline six inches and check again.
If you start in a corner and the corner is not perfectly square, you will end up with unparallel joints throughout the room and possible small unstable slivers at the walls.  After laying out from the center and knowing which lines are square, you can start wherever you want but you can't just start from the corner without doing some survey work.

I agree with freedom1st. Always start at the center of the room.
  Once you find the center, mark it with cross lines and start by placing a tile in each corner of that centerline. Lay a row of tile on the floor in each direction up to the walls in order to get an idea of the outcome before actually laying it.  You don't want one side of the room to have a full 18" tile and the other side having a shortened tile, which is what would happen if you start in a corner. This would make the room appear uneven and it just plain looks bad.
Use thinset mortar mix and a notched trowel that is at least a 1/4 inch notch. Your supply store can recommend the proper size trowel for your 18" tile.
Remember, good planning always determines a better outcome of your project.

18 in. Is 1 1/2 ft.  You'll probably have 1/4 in. Of whatever adhesive you're using.  

Start in the corner of the room and work your way out.

Put the adhesive on the bottom of the tile and put the tile in the corner. (this tile is 18"x18")
put your adhesive next to it. (probably at most lay 1/4 in. Of it) then lay another tile next to it.  

I made a drawing of how the tile should look.  If you give me your email, I can send it to you b/c I can't get it to upload onto the site correctly.
Thanks.