I have a grid and dropdownlist.I want to filter values in a grid by the selection of dropdownlist.How Can i do this? My code is like this

<asp:DropDownList ID="DDLVisitedVol" runat="server" AutoPostBack="true" DataSourceID="DsVisitedVol"
            DataTextField="VisitedVol" DataValueField="VisitedVol" 
            Width="244px">
        </asp:DropDownList>

 <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString=""
           ProviderName=""
            SelectCommand="SELECT [ID],[UserName], [Email], [visitedVol] FROM [HitTracker] where visitedVol=@VisitedVol ">
        <SelectParameters>
                <asp:Parameter Name="VisitedVol" Type="String"/>
        </SelectParameters>

How can i pass the selected value of dropdown list to @VisitedVol . If anyone knows pls help me. tHANK YOU