Normal heavy shields do 1D4, normal light shields do 1D3.
You can not bash with a tower shield or a buckler.
You can put spikes on a shield (listed under armor in the PHB) which improves the damage die by one. So a heavy shield with spikes does 1D6, and a light shield with spikes does 1D4.
Now, there is also the "bashing" enhancment in the DMG. That also imporves the shield's damage by one.
Now, you can enchant shields and spikes all you want, but they are seperate. For example, you can have a +2 heavy shield with +3 spikes on it. The +2 effects AC the +3 effects attacks and damage.
As a result, I don't allow "bashing shields with spikes" to do 1D8 / 1D6. In other words, I don't let the two things stack. I think the enchantments on the shield do not effect the damage if you are attacking with the spikes.
Others allow it. Ask your DM.
Anyway, it's all in the PHB (though the 'Bashing' enchantment is in the DMG).
Have fun!
-Tatsu