I have an API user name and API Password and a very long string of letters (I'm not sure what they are)

When I'm creating a new form, do I need to enter these or just use the user name and password (and email?) and this will be added automatically?

You have to login to your account before creating the form. The API will be used when you are using Jotform outside from the website and/or when doing third party integrations. 

This is our API Docs: http://api.jotform.com/docs/.