When developing a style guide for a website, you need to think about the brand that the website will be representing. For example, if you are developing a website for a corporate legal firm, the look will need to be clean and professional. If, on the other hand, the website is for a tattoo parlor, then the look will more likely need to be vibrant and funky.
A style guide is essentially a document that lays down the styles, colors, fonts, images and other design attributes that will be used on a site. It helps a company by making sure that the look is always consistent. Your style guide will need to cover a variety of points, including overall look, colors to be used, fonts to be used (different fonts may be used for headline copy and body copy), and examples of the types of pictures that can be used.
It should also include examples of unacceptable web design, font usage and the like.