First question: "Why am I building this web site?" Are you building the site to make money? To share information? Or to build community, to keep in touch, or other collaborative effort?
If you are planning a commercial venture, consider tips on business planning, such as this magazine article.
The most basic tool to get started, after your business plan, would be a site map.
A site map is a diagram that represents the hierarchical structure of your site. The site will ultimately be rendered in Hypertext Markup Language (HTM), but this blueprint provides guidance on the overall navigation and structure.
To get started creating a "site map" of what you envision, read this...
You can use a tool such as this, http://slickplan.com/ or http://www.gliffy.com/
A site map is a planning aid that maps out the pages and navigation/structure, to help organize and categorize you content so that visitors can achieve the end goal of viewing your site.
Of course, you could also just use paper and pencil, which lets you free-think with little up-front learning.
Once you'd got your business plan and site map, you should decide if your site can be static (just HTML files) or dynamic (frequent content updates or interactivity, requiring more sophisticated technology such as a content management system (CMS) or actual computer code.
If you have relatively static (unchanging) content, the old method of updating an HTML file manually and uploading it to a server may suffice. This low-tech, low-cost approach can get a site launched fairly quickly.
if your content needs to be able to be updated regularly (daily, weekly, ad hoc), with different people adding content, or content that is updated automatically through the use of automated tools, a CMS might be the way to go.
Another advantage of a CMS -- no need to master HTML. In many organizations, content authors rely on web developers to put the mark-up formatting codes in place to make content web-ready. With a CMS, the content author/owner can access the parts of the site specific to their own content, and publish, often directly to the web site. Sometimes workflow is used, to enable third-party review and approval.
Finally, a CMS makes applying design standards more simple. With static pages, there is great flexibility, but one often relies on templates to ensure a consistent look across a site. With a CMS, non-authorized users cannot make changes to controlled areas.