Get the All Communities feed to see a list of all public communities to which the authenticated user has access or pass in parameters to search for communities that match a specific criteria.
You can derive the URL from the service document. Find the value of the href attribute of <collection> element that contains the <atom:category scheme="http://www.ibm.com/xmlns/prod/sn/collection" term="public"/> element.
Use one or more of the input parameters to narrow the search. Separate multiple parameters with an ampersand (&).