A good starting point for planning the future of your website is to analyze what you already have. To some extent we are doing this all the time. That is how new projects happen. However, a more formal approach helps to better inform your decision-making throughout the web project. There are two ways to better understand your current website: qualitative and quantitative.
Boag, Paul. Digital Web Magazine (2008). Articles>Web Design>User Centered Design>Audience Analysis