Many elements go into the design of a web site.
Obvious elements will include the graphic style and layout, navigation, text copy and so forth.
Not-so-obvious elements will include things such as color scheme matched to target audience, pages coded to be compatible with the widest assortment of popular web browsers, text and "behind the scenes" tweaks to improve search engine visibility etc.
Before attempting the design of a site, try to know a few key things: What is your target audience and their demographics? What *exactly* are you trying to achieve with the site? What sets your products / services / viewpoint apart from your competitors? Have you seen any sites which mirror what you are trying to achieve? Answer these and you will be well on your way...