Ontology-nameSpace

[hlel emna]

I tried to create an ontology with jena

String ns=??????;

OntModel m = ModelFactory.*createOntologyModel*();

OntClass autt = m.createClass(ns+"Auteur");

      OntClass thaut = m.createClass(ns+"Theme");

      OntClass motc = m.createClass(ns+ "Mot-Clé");

………….

how I can chose the namespace: