Most of SGD's webservice calls require a session cookie. This is in order to authenticate the user making the call, and verify that they are authorised to do so. There isn't a way to get information about sessions owned by a user without providing a session cookie to prove you're authorised to have this information.
When using the webservices you would normally get a session cookie by using the ITarantellaWebtopSession.authenticate() method to authenticate as a particular user (see http://docs.oracle.com/cd/E41492_01/E41499/html/com/tarantella/tta/webservices/ITarantellaWebtopSession.html#authenticate(java.lang.String, java.lang.String, java.lang.String, java.lang.String) for the documentation on this in 5.10). You can then provide this session cookie to the other webservice methods you wish to use.