Take a look at the Invoices starter solution (It's in the file menu.)
You may be able to use that or at least get some ideas.
We set up an invoicing system in filemaker 5 (purchase orders are very similar) and were able to convert it over to Filemaker 10. The Invoice table with portal to LineItems table is the same in both versions. You just have to learn your way around Manage | Database to figure out how to set it up.
Feel free to ask questions here. There are also books and tutorials available that you can use to help update your information if you wish to invest the time and money.