Step 1: Conduct Needs Analysis
Do you know why you're building an intranet? Is it meant to solve problems, improve efficiency, or just to bring your organization into the 21st century? Or are you just setting out to improve the existing intranet that your company's Vice-President's best friend's nephew designed in 1997? All of these are legitimate reasons, but you need a breadth and depth of understanding in respect to all issues before you can go any further.
Assess the current site (if it exists) and understand where it is hosted, who was originally responsible for creating it, what the history of the site is and who is currently responsible for maintaining it. Create a site map at this time so you're clear about how everything works (and you can refer to it when you're creating the new one).
Consider preparing a survey for all stakeholders that solicits their opinions on content, functionality and design (including usability issues). Get feedback on current intranet (if it exists) as well as identifying "Needs" for the new intranet. Remember that the intranet should be for employees, and for making their work lives easier; it's not exclusively for managers of various departments and lines of business. Make sure to solicit feedback from the people who will use the intranet to increase the likelihood that it will provide them with utility.
Aggregate all the information gathered during the Needs Analysis and prepare a document that states the particulars of the current site situation and what needs have been identified for the new site.