You have to understand WHY you are creating a website. What is the purpose of your website? Is it to promote your business by increasing awareness? Or do you want to sell products through an online store? If you are a charity you might want to provide member-only content, separate and hidden from the public face of your site. Knowing the purpose helps you choose the right theme for your build, since each theme comes with a certain set of functions. For example, a theme template suited for membership has the right widgets for member management, protected pages, etc.
Next, you need to identify WHO the site is for. You may have multiple audiences, or one main audience and a secondary audience. When branding and writing your website content you want to write for your audience, not yourself or your boss!