All depends on what you want to do: editing your own invoices or keeping
track of the invoices you receive.
In case of creating your own invoices, this wil depends on the local
(eventually legal) "needs".
Some indications: begin with creating some tables: one for your
customers/furnishers, one with current tax-levels, one products-table, one
with details of your "sales" (productnumber, quantity, ....) and decide what
kind of information you need at the end.
The lay-out of your invoice is only the final work.
You can find some indication the example NorthWind dbase which comes with