Simple Search

I have a simple search going on:
Search Criteria:
By FirstName
By SurName
By Employee No
By WorkPhone No
Input box
user enters whatever criteria, selects the criteria from the dropdownlist and idpslays the result in the gridview.
But the problem is that if I input 'Douglas' and Select the 'By FirstName' it then pulls data from the LastName field as well.
My Code:
<asp:SqlDataSource ID="SQLForceDirDataSource" runat="server"
                                    ConnectionString="<%$ ConnectionStrings:ForceDIR_Connection %>"
                          ...