Do you really have one database file for contacts and one for Inventory? That's doing it the hard way, especially when you are new to FileMaker. Instead, go to Manage | Database | Tables and define different tables in the same file, then link them in relationships all in one file.
I'm guessing from what little is in your first post that you want to set up a system for managing the sale of products from your inventory to your customers. The Invoicing starter solution that comes with FileMaker is already set up for this. You may be able to use it, or adapt it to your needs, or it may just be a good working example you can explore in order to get useful ideas to use in your own solution.
The typical invoicing database is set up like this: (I"m using the table names that I recall from the filemaker 12 and 13 starter solutions so this will apply to both the starter solution and any file you might create "from scratch" to do the same thing.)
However, I'm going to use my own field names as those used in the starter solution strike me as very unwieldy.
Customers::__pkCustomerID = Invoices::_fkCustomerID
Invoices::__pkInvoiceID = InvoiceData::_fkInvoiceID
Products::__pkProductID = InvoiceData::_fkProductID
With this setup, you can put a portal to InvoiceData on your Invoices layout and use it to list items purchased on that invoice. And the _fkCustomerID field can be used to link a given Invoice to a customer record so that you can show data from the customer table on your invoice table.
That barely scratches the surface of what you need to make this work, but hopefully it can get you started in a better direction.
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
Thanks for your time!
I still don't understand any of this, so I think I will try another program instead.
Have good day!
Before giving up and moving on you should read the online help and manual. I understand Phil's post might seem hard to understand if you're a complete beginner, but with a little work you'll understand his excellent answer. A relational database is very powerful and worth pursuing. Perhaps google relational database theory as well. There are a few basic core concepts that, once grasped, will smooth the path ahead.