Hi I'm new to stacking cakes and I was reading that I need to use dowels and cake boards to support the upper tiers. My plan is to use 1 9x13 and 2 8 in square pans. I'm looking to make something like this http://chanisdelectables.blogspot.com/2013/06/tefillin-cake.html but covered in fondant. I'm a little bit confused with what type of dowels and cake boards to use. I read that I need to use cake boards in between the layers, but am unsure what type of cake board I need.

When stacking one cake tier on top of another, the top cake should be on a cardboard square that is the same size as the cake.  I usually put one under each tier, even the bottom, so I can then place on the presentation board without the frosting mess.  There are precut cake boards (the cardboards) that are coated on the cake side  at craft stores and cake supplies.  Some people do cut their own, and some even use foam core for larger cakes.  You would need to cover these with plastic wrap, as they don't have that food safe coating.

You will need to place wooden dowels, bubble tea straws, or "hidden columns" in the bottom tier to support the top tier.  Determine where the top will be, then make sure the supports are within that area.

There are many videos on YouTube that show how to properly stack a cake with supports.

The 9x13 needs to go on a cake drum or foamcore base board. You would need an 8" square cake board to build the 8" cake on and to decorate it. You would mark where the 8" is going to go on the 9x13 layer (useful to have a spare 8" sq board for this). Insert 5 dowels - cut to the height of the 9x13 tier - inside the 8" square you have marked, one near each corner and one in the middle of the square. Add a little BC or royal icing on top of the dowels, place the 8" cake on it's board on top of the dowels.