Hello,
Let's say that as part of your web application you have a page where a user will have access to his/her tasks.
So what you need to do is when the user visits that page (i.e. jsp page) the servlet/bean (or whatever) associated with the jsp will have to retrieve the tasks for that user and give them to the jsp to display i.e. a table/grid with all the tasks of that user and the option to select a specific task.
Retrieving the tasks is accomplished by communicating with the human task service and when the user selects a task (i.e. by click a button right by the task entry of the grid, or the grid row etc) you will present a jsp/html/ajax (or whatever) page representing that task. The details regarding the task i.e. its parameters, name of the task etc will also be retrieved from the task service as well as the signal for completion. The button that completes the task on the page that you will present will have to call the api related to completing a human task.
Having this idea in mind look at the api code related to communicating with the human task service (for getting the tasks for a user, starting the task, completing the task),
http://community.jboss.org/people/bpmn2user/blog/2011/04/03/jbpm5-human-task-api-usage-example
http://docs.jboss.org/jbpm/v5.1/userguide/ch.Human_Tasks.html#d0e3412
access task variable - http://community.jboss.org/message/603579
another thread - http://community.jboss.org/message/606050