Of course it's possible!

If you would like a suggested formula:

=SUM($A$1:A1)

Now, if this doesn't work for you, how would I know that, with all the
information that you gave us?

Every  month I have to change the sales amount in my worksheet.  I would
like this amount to be added automatically to the total for the year sales
amount in the next cell.  Is this possible?

right click sheet tab>view code>insert this>modify a10 to your cell>SAVE

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address <> "$A$10" Then Exit Sub
Target.Offset(, 1) = Target.Offset(, 1) + Target
End Sub

I guess I should explain myself a bit better.
Every month I enter the actual sales amount in Cell C13,  then in Cell D13 I
enter the cummulative sales amount for the year (which I calculate manually).
The following month I enter the new actual sales amount in Cell C13 and I
add this amount to the previous cummulative sales amount from the previous
month in Cell D13.   Can D13 be updated automatically with a formula every
time I enter a new amount in Cell C13?