Try the helloworld portlet examples jboss website jboss portal wiki page.
You have simple and first portlet idea. Then u have to concentrate on the portlet api and some examples to try.
you can keep your portlet in a separate webapp. explode that to your deploy folder, and you can have very fast turnaround when developing (you only need to change your local resource and java files; when changing classes and jsps you need to touch the web.xml, but no redeploy is necessary).
But even keeping it as one WAR and redeploying that is sufficiently fast.