In the triggers section add the trigger "Number of items with a specific column value":
If the number of items where [itemType] has a value of  is [>] a value of 
and in the calculations section use "based on items with a specific value":
total of [Totalprice] Where [itemType] has a value of ] [times] [0.10]
for a 10% discount on all items with an item type of 1.