You would decide how you wanted to set up your css file (external or in the head).
You would then open the CSS box and click on the add new style symbol, which would ask you whether you wanted the style in an external style sheet or in the page.
Once you got to the style options you would choose change properties of a tag, and then choose the <body> tag.
Once you have done all that, it will click up another box asking you what styles you want. In this case you would choose Background, and navigate to the image you want as the background, and then Background-Repeat: no-repeat.
Or, of course, you could look up the relevant css and add it into an external style sheet or in the head manually.
It may be boring but you would find it well worth your time going through the tutorials available within Dreamweaver, as well as taking the time to learn a little bit about how css works from some of the online resources.