to integrate existing site to portal and portlet.
1. Divide the whole website in applications. As portlet are also know as application which is designed to do some specific task. Every portal page can have multiple portlets so divide accordingly.
2. In portal you dont need to worry about the header , menus and footers as portal have concept of theme and skin. So you need to develop theme and skins in between that your portlet application will work.
3. if you are using RAD or RSA or any eclipse each can create sample portlet application from which you can understand the portlet and its struture.
4. portal have the concept of action phase and render phase so you need to divide existing site accordingly.
a) render phase : this phase is responsible for just to display the content.
b) tender phase : this phase is responsible to update those attribute or variable which will affect the render phase or the business logic to display content in render phase.
I hope now you got some what idea.