Hi,
I'd like to recommend the "what is a Bdoc?" blog post, by Stephen Johannes. You will discover all our secrets to get to know SAP CRM. Begin with the first one: go to http://help.sap.com. Then navigate to SAP CRM Best practices.
Or you can also use this direct link: SAP Best Practices for CRM
You'll find everything you need to know to setup CRM / ECC integration.
Regards,
Nicolas Busson.