To develop a Portal or to convert your existing application to Portal you need to have a Portlet Container just like some
servlet container.
Now as I told you there are some open source Portal Servers available in the market from Apache, Liferay and so on..
Thing is if we take Liferay for instance then it not only provides you a good framework to develop your Portlets but also provides a number of Portlets like Blogs, Message Board and so on which can be used as it is or can be customized. Also with some customization you can change the look and feel and get your website up and running.
Liferay is free so if you decide to use it liferay.com will be the best place to start.
If you need any help in deciding the platform or for getting more inputs on this just let me know.
