If you are using the query wizard, then just insert the column you want to total along with the data you want to group by, and then click the Totals icon (kind of looks like a backwards 3). That will add a Totals row to your field columns. It has a drop-down box where you can choose group by (default) count, sum, etc. for each field. Choose Sum for the one you want to total, and leave the others as group-by. (You may have to experiment on your field selection and group-by options.)
If you are using SQL, then here is a simple example:
SELECT tblOpenInvoices.supnum, Sum(tblOpenInvoices.InvAmt) AS SumOfInvAmt
FROM tblOpenInvoices
GROUP BY tblOpenInvoices.supnum;
Note: you can change the name of the summed field by inserting the name after AS
I hope this helps.
Danhauer