Experience, training or licensing may be needed
Many courses exist (many of which, logically, are offered online) where you can learn the language of website creation and can learn about the details, like how to set up shopping cart systems, security concerns, etc. You will, of course, need to learn about each company you design for. What is the atmosphere of the company that you need to reflect in the website design–is it wild and contemporary, meaning brilliant colors and fun graphics? Or will more classic colors like black, navy blue and maroon be more appropriate?