1st off, and I don't mean to scare you, but developing such a piece of
software involves a lot of work.  I typically advise my clients to simply
purchase an accounting package (Quickbooks, Dynacom,....)  cheaper, well
developped....

That said, no, you do not want seperate table for each month.  Simply have a
master table that has a date field.  Then you can build your queries to
filter by day,  month and/or year.  You'll want the ease of use for other
functionalities such as year end reports...

Also, take a look at

http://office.microsoft.com/en-us/templates/CT101426031033.aspx

There are a few sample databases that you can inspire yourself from.

A table per month!? Yuck. Not in Access; it might (barely) make sense in some
older programs.

No, you have a One (customer) to Many (invoice) relationship. You need two
tables, with a CustomerNumber as the primary key of the customer table, and as
a foreign key in the invoices table. I would discourage the use of your
current composite invoice number; fields should be "atomic", having only one
piece of information. You're storing three (customerID, year and month);
that's hard to maintain and inflexible (suppose you someday need to send a
customer an extra invoice during a month? Redesign your table to accommodate a
suffix to the invoice number!?)

Take a look at these resources, and at the Northwind sample database (which
handles customers, sales and invoices) as an example.

Jeff Conrad's resources page:
http://www.accessmvp.com/JConrad/accessjunkie/resources.html

The Access Web resources page:
http://www.mvps.org/access/resources/index.html

A free tutorial written by Crystal (MS Access MVP):
http://allenbrowne.com/casu-22.html

MVP Allen Browne's tutorials:
http://allenbrowne.com/links.html#Tutorials

            John W. Vinson [MVP]