Based on what is listed above, you need to create an event handler for the OnSelectedIndexChanged event - see below;
<asp:datagrid id="dgListaDistributori" runat="server" cssclass="tblDGStandard" width="100%" headerstyle-verticalalign="Middle" headerstyle-height="20" showheader="True" autogeneratecolumns="False" onpageindexchanged="PageGrid2" allowpaging="True" cellspacing="1" cellpadding="0" borderwidth="0px" xmlns:asp="#unknown"> OnSelectedIndexChanged="gListaDistributori_SelectedIndexChanged"></asp:datagrid>
In your code behind you can then access the cell value as follows;
Dim strMyValue as String
strMyValue = dgListaDistributori.SelectedItem.Cells(1).Text