Retrieve Data From SQLDataSource

Background:
I am using Visual Studio 2005 Standard SP1 to create an ASP.NET website that accesses an SQL 2005 database.  I am using vb.net as well.
I am passing an ID in a query string from one page to the next, where I retrieve it using an SQLDataSource.  The data will only be one row; it is returning customer information.

Problem:
I need to be able to get specific fields from the SQLDataSource and populate some textboxes.  Honestly, I'm not really sure where to begin.  For the sake of argument, let's say that I am working with this:
'My Textbox
Dim LastName As Textbox

'My SQLDataSource (filtered by query string using the customer's ID to only get 1 customer's information at a time)
SQLData

Question:
So, how would I go about retrieving the lastname field from SQLData and inserting it into the textbox LastName?

Thanks,
    J'Tok