.ASPX

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder2" runat="server">
    <asp:Label ID="Label1" runat="server" Text="Username:"></asp:Label>
    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
    <br />
    <br />
    Password:<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
    <br />
    <br />
    <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Vai!" />
    <br />
</asp:Content>

.CS
protected void Button1_Click(object sender, EventArgs e)
        {
            //Stabilisco la connessione al server
            string conn = "Server=PC1319; Database=kitkits; User Id=sa; Password=1mp0ss1b1l3;";
            SqlConnection con = new SqlConnection(conn);
            SqlCommand select = new SqlCommand("SELECT COUNT(*) FROM Cliente WHERE username ='"+TextBox1.Text+"' AND pwd='"+TextBox2.Text+"'",con);
            DataSet ds = new DataSet();
            SqlDataAdapter da = new SqlDataAdapter(select);
            da.Fill(ds, "Cliente");
            string accessoeseguito = ds.Tables["Cliente"].Rows[0].ItemArray[0].ToString(); //se 1 success, se 0 fail
            if(accessoeseguito=="1")
            {
                Session["id"] = TextBox1.Text;
            }
        }