To use the API, you will need to know your CID and PASSWORD.
You can retrieve both of these details form the MANAGE FILES area on the left menu, under classic FTP details. The password to use in API calls is the FTP password. The CID to use is the FTP username, without the CID prefix. ie, just the number.
The CID and password are unique to each website. You should pass these details through as POST parameters to ensure best security of your data. GET will work, but your password is subject to being stored in any proxies between you and our server.