Gathering and structuring content is key to knowing what to design. Content inventories, card sorting, and sitemap creation based on balancing user and project goals defined in the Discovery phase.
Gathering content can be like herding cats, but one of my favorite parts of a website is creating structure with sitemaps and navigational options. I like to use competitor research to find out which navigational terminology are used for certain types of websites. I organize content into logical groupings that help people find what they are looking for.