You first need to use Delivery Manager to set up your default warehouse, carrier configurations and carrier 'service groups'.
If you are new to Delivery Manager, you can find out more about it here.
You need to work through the following before using the Metapack Shipping API with Salesforce:
A 'service group' is a collection of services (e.g. 'Next Day'), and a service is an individual carrier service belonging to a group (e.g. 'Royal Mail 1st Class' within 'Next Day').
Ensure that you can create and allocate Consignments, and create the documentation for them, using the following Shipping API call: despatchConsignmentWithBookingCode.