it's pretty easy:

  

     cartodb.createLayer(map, 'http://documentation.cartodb.com/api/v2/viz/2b13c956-e7c1-11e2-806b-5404a6a683d5/viz.json').addTo(map)
       .done(function(layer) {
            
          var sublayer = layer.getSubLayer(0);
          sublayer.set({ 'interactivity': ['cartodb_id', 'name'] });
    
          var i = new cdb.geo.ui.InfoBox({
              layer: layer,
              template: '<h3>{{name}}</h3><p>more info</p>',
              width: 200,
              position: 'bottom|right'
          });
          $('body').append(i.render().el);

      });

I'd recomend you to take a look to The Hobbit example: https://github.com/CartoDB/cartodb.js/blob/develop/examples/TheHobbitLocations/js/app.js