Many people think this is where you jump into your web editor and start building, but the best sites start with a plan and start that plan even before the first wireframe is built.
Your design plan should include:
Details about the information architecture
The planned structure of the site
A site map of the pages to be designed and built
And technical details like: if scripts or Ajax will be used, whether there will be a server-side language like PHP in use, if you need a shopping cart and so on.