Morning Shanzalot. I must first declare that I'm not a fan of Access - its a brilliant program, but much to complicate for many day-to-day tasks. The best use I've seen of it is as a front end to an Oracle database, but I digress.
Excel is an outstanding tool for simple databases, and when linked to Word you can easily create all the forms you want. A simple precis of the process (as I recall) is:
1. Create the database in Excel (each column is a field (make sure you put a column title for each), each row is an entry - if using MS Office 2010 save the file as Excel version 2003 as the new format didn't work for me!
2. Create a form in Word. This is a mail merge, but select the 'Letter' options and use 'Existing Data' as the source. Essentially you create whatever form you want in word and then insert fields that are linked to the Excel spreadsheet. You can then select which records you want to turn into invoices.
I know this seems rude, and its not meant to be, but from you're question I sense that you've very little experience with Access and though what you want to do is not especially difficult, you might find the suggestion above is easier. Certainly it is the way I would do it.
Hope this helps - even if it isn't the direction you wanted to take.
Regards
Peter