First of all, I would suggest that in your 12 monthly sheets you have
the totals on the top row - they can be set up like:

=SUM(D3:D500)

for column D, assuming you have a header row (which now becomes row2)
so that your data starts in row 3. By having a range up to 500, even if
you don't use 500 rows, this ensures that you sum all the data in that
column.

Then in your summary sheet, the formulae needed will be very similar.
Assuming you want 4 totals going across for each month, then in column
A you can enter January, February, March etc going down the column. In
column B on the same row as January, you would have a formula like:

=January!D1

where January is the name of the sheet - if the sheet is called
something like Jan 06, then the formula would be:

='Jan 06'!D1

Note the apostrophes - these are needed if the sheet name has spaces in
it.

Let's say that your other totals are in columns F H and J of the
monthly sheets. You would need these formulae to bring the January
totals into columns C D and E of the summary sheet:

C: =January!F1
D: =January!H1
E: =January!J1

For February you would have 4 similar formulae, but you would change
the name of the sheet. Similarly for all the other months.

Hope this helps.

Pete