First you really should read JSR168 spesification. It is not very long and it describes how portlets plug into portal. Usually I don't like specs but this one is actually very good and every portlet developer should read it - not just those who develop portals.

Then you should download some helloworld portlet (net is full of them, i got thousands of references in google) and try to deploy that one into portal.

Next thing is of course move on to more complex portlets. Just read alot what others have done. Look into portlet sources of jboss portal itself.

Usually when I research new things before presenting them to our clients as possible platforms/solutions, I do some real applications - for me and my friends mostly. Currently I am recoding my wifes netshop that has been up 2,5 years without much development. It is currently done in plain jsp and proprietary persistence layer. I targetting to JSR168 portlets + hibernate. It is about 25% done now and I hope I get it online next summer :).

In some point you need to decide are you going to use just bare JSR168 or are you going to do portlets targetted to jboss portal only. This is hard decision to make. I have made compromise that allows me to deploy to multiple portals and still use portal implementation spesific stuff.

In this case I have splitten my project into 4 parts. I have shared java code that contains mostly hibernate and persistence stuff. Then I have shared portlet project that can be deployed in jboss, jetspeed2 or any other fully jsr168 complant portal. And then I have one portlet project for each portal implementation that uses implementation dependent stuff like user registration, layouts/themes and portal structure deployments. Yes I know that this could be done with build scripts too, but I have no time to implement complex build system.

Currently my application runs on both jboss and jetspeed2 and I haven't decided yet which one is used in final production deployment. JBoss have great and flexible layout/theme system but Jetspeed2 is more complete on security side and profiling that is also very important for me. I wish I could have both :)

I mostly hate writing deployment descriptors. Fortunately I have RAD6+Portlet Development Toolkit available that makes descriptors almost transparent to me. This however is commercial software. I don't know any complete OS JSR168 portlet development environments (ie plugin for eclipse). Hopefully somebody else can help with that.