The filemaker go development guide is a good place to start:
I'd also recommend using some of the starting solutions and default filemaker "touch" themes. These are things designed to work already for iOS that will give you a good idea for some of the usability things.
Outside of that, the normal course of action is to adapt your existing layouts for use on iOS by making a copy of them, and having a script that redirects iOS users to that "interface"
Do you mean three files??
First step might be to get the three tables in one file. Will be easier to transfer to the ipad then, and the logic will all be in one place.
I also suggest just creating some new files from the starter solutions to see how they have created layouts in the right size and orientation for iPad
FileMaker Training series Basic is also free and a good way to start if you are really new.
In principle, you want to create a new order, populate that with the details of a chosen customer, then see the cards available in a portal, to add a number for each one, and then transfer that detial to a line items table, linked to the invoice record by its ID
I have the 3 tables in one file
Form follows function:
You get a phone call / email / letter from a Client
Go to that client's record
Create a new order: go to a ordering layout based on clients, showing client and order data in header.
==> cartesian portal showing all cards with filter, empty portal showing order lineItems
Click on cards to add them as lineItems, move to the quantity field after adding and select the value within (autoenter with minimum order quantity)
once you got the flow approuwife'ed you can start layouting