In order to create a list using our API's, you would want to do a POST HTTP call to the list URI on our API servers, sending over the appropriate XML.
Our list URI is https://api.constantcontact.com/ws/customers/(USERNAME)/lists and this is where you would do the POST call to. You can also find the XML structure that you need in order to create the list here.
Once a list is created, it returns you the list XML, and you can then use this to get the List ID, which you would like to add contacts to the list.
If you have any questions regarding this, please feel free to ask here.
Benjamin Soder NOC Analyst Constant Contact
View API documentation, code samples, get your API key.