To start with you need to understand the JSR specification. If you have knowledge and experience in
J2EE then it should not take you long to understand Portlets.
Next thing is that just like
Servlets need a Servlet Container to execute, similarly Portlets require a Portlet Container.
There are many different Portlet Containers available but few popular ones are provided by IBM, Liferay,
JBoss. They provide Portal Servers.
Job wise currently the market is good for Liferay and IBM Websphere Portal servers.
You can visit the following
URL to get more clarity and understand things. Let me know if I can be of more help.