How to retrieve selected value of dropdownlist filled from datasource in asp.net?

Code:
        Dim str1 As String = "SELECT UserName FROM Login WHERE Category = 'Property'"
        Dim cmd1 As New SqlCommand(str1, conn)
        Dim myreader1 As SqlDataReader
        conn.Open()

        myreader1 = cmd1.ExecuteReader()

        DDLCat.DataSource = myreader1

        DDLCat.DataValueField
...