The following example group an list by year, month. It counts how much records the groups has and sums some values from the group (in this case is money).
var orders = from ord in Lista
group enc by new
Month = g.Select(n => n.DateOfPayment.Value.Month).First(),
TotalAmount = g.Sum(n => (Decimal) n.TotalAmount),
CountOrders = g.Count()