Hi Ramiro,
I would recommend you to start easy using Bitronix transaction manager. Set it up to run in Tomcat as per http://docs.codehaus.org/display/BTM/Tomcat2x and fill gaps with http://www.plugtree.com/jbpm-5-1-in-tomcat-6/ The last one talks about the JBPM Console but it is pretty much what you have to do for your own web application.
Cheers,
John.