You should begin by thinking of the purpose of the website, and how it will fulfil this purpose.
You need to decide what Software or Tool you'll use to build the website in, because this determines how you'll present the content, and update the site.
You should then collect the content for the website. This includes the text (called 'copy') and the media, such as photos, videos, etc; as well as any other info and elements.
Once you have an idea of what the website will have in it, you must decide on the layout; how pages link and relate to each other.
At this point, you should also decide who will do what work for the website - building the website, and maintaining it when it's done.