populate label/textbox from dropdownlist

I've been trying to find a solution to my problem for about 2 days now, and have found nothing.  I am new to .net and not really sure how to accomplish this. 
I have 4 tables in a database, Vendors, RepairCos, Cons, ConRepairHistory.   What I'm trying to do is make a page called SendOutForRepair.aspx that has a insert item template where the first control is a dropdownlist that is attached to the Cons table (got this figured out) and is populated with the Serial Number field.  I would like the selection from the dropdownlist to populate the 2nd control (VendorID) that is also from the Cons table.
example record from Cons Table:  Serial number      VendorID
                                        &n ...