Start by downloading the QuickBooks SDK:
Install it, and look at the file named:
- QuickBooks Web Connector Programmers Guide,
QBWC_proguide.pdf
It's about 100 pages, and a spec/guide of what you'll need to implement since there's not much else out there pre-built for RoR.
At a (very) high level, you're going to create a SOAP service which listens for HTTP requests. There's a number of methods you'll need to implement:
authenticate(...)
- QuickBooks will call this to authenticate to your service
sendRequestXML(...)
- QuickBooks will call this to ask "What data do you want to exchange?" at which time you respond with a qbXML request telling to add an invoice, or add a customer, or give you a list of customers, or etc. etc. etc.
receiveResponseXML(...)
- When QuickBooks has finished doing whatever it is you told it to do in the previous step, it will call this method to let you know what happened (e.g. was whatever you told it to do successful or not).
There's more details here that you should read:
The QuickBooks SDK also includes examples in C# and Java for this. Look for the Web Connector-specific ones, not the COM/QBFC examples (those aren't relevent to you).
You might also look at this for reference/ideas (it's in PHP, but might still serve as a reference):