In Salesforce, you would need to create a connected application to get the API key (In Salesforce's term it is consumer key and secret).
Using this key a Shopiforce App can access Salesforce by its REST service.
In order to obtain Salesforce credentials you need to create a Connected App
In Salesforce go to Setup -> Under Build section on the left -> Create -> Apps -> New Connected App
Now you can click Save and that's all you need!
Now you should see this screen:
In Order to obtain Security Token please click on your username in upper right corner -> My Settings -> under Personal tab you will have Reset my security token. Once requested it will be sent to your Salesforce email address