Populate textboxes from a dropdownlist

Question

Hi, I am quite new to ASP.Net and am having a problem with a dropdownlist. The user will choose a site number from the dropdownlist. This should then automatically populate the textboxes called site_name, site_address, site_county and site_postcode with data from the following SQL command:

SELECT site_num, site_name, site_address, site_county, site_postcode FROM SCSite WHERE Site_Num = <value from dropdownlist>

I believe that I should be using OnSelectedIndexChanged, but am stuck with the C# code behind to achieve the population of the textboxes. Below is the code for the relevant section of the main page.

Any help would be most grateful!

<td>
     <asp:DropDownList ID="Top_Site_NumDropDown" AppendDataBoundItems="true" runat="server" DataSourceID="SqlDataSourceSite" DataTextField="Site_Num" DataValueField="Site_Num" AutoPostBack="true" OnSelectedIndexChanged="Top_Site_NumDropDown_SelectedIndexChanged" SelectedValue='<%# Bind("Top_Site_Num") %>' >
                            <asp:ListItem Value="">Please Select</asp:ListItem>
                        </asp:DropDownList>
                </tr>
                <tr>
                    <td>Site Name</td>
                    <td><asp:TextBox ID="Top_Site_NameTextBox" runat="server" Text='<%# Bind("Top_Site_Name") %>' Width="95%" />
                    </td>
                </tr>
                <tr>
                    <td>Address</td>
                    <td><asp:TextBox ID="Top_Site_AddressTextBox" runat="server" Height="4em" Text='<%# Bind("Top_Site_Address") %>' TextMode="MultiLine" Width="95%" /></td>
                </tr>
                <tr>
                    <td>County</td>
                    <td><asp:TextBox ID="Top_Site_CountyTextBox" runat="server" Text='<%# Bind("Top_Site_County") %>' /></td>
                </tr>
                <tr>
                    <td>Postcode</td>
                    <td><asp:TextBox ID="Top_Site_PostcodeTextBox" runat="server" Text='<%# Bind("Top_Site_Postcode") %>' /></td>
                </tr>