The process of site design is started by working with the client and potential end-users in analysing the requirements for the site — this is the case for even a one page site.
What does the site aim to acheive? Who does the client intend as end users? Who are its likely end-users? What do they aim to acheive from the site?
Answering these questions, you begin to get a better idea of the focus of the design process.
To establish an information architecture for the website, you will need to analyse the existing content on the website, or within the organisation.
This, together with the user research will help create a taxonomy (classification) of the content and, from that, a navigational schema, and maybe some broad ideas of the layout of the navigation.
This can be done through a process of analysis