You need access to the website's source folders. Usually an ftp location. If it's a .Mac website you can just copy the files off the mounted iDisk folder.
In any case you will need an administrator access to these locations. Your friend may have the access detals, or the administrator of the site, eg the isp.
If you don't know what I'm talking about then you might need a beginners course for web authoring, or to latch on to a physical human being who knows this stuff. This could take a long time with discussion boards.
You can access the source code of the site if it has been created from static html pages. The quick and dirty way is to browse a page and select view source from the View menu in Safari, or equivalent in another browser. You can save that text file to your hard disk with a .htm or .html suffix and then open it in Dreamweaver and start experimenting. But you won't be able to upload back to the website with this method.
You also won't have all the linked images. You can drag and drop them to your hard disk from your browser and relink them to your html page in Dreamweaver.
Of course this is a slow and tedious process. There is software which will download whole sites to your hard disk eg Webdumper
http://www.maxprog.com/WebDumper.html
There's a lot to learn. A book might also be helpful in getting you set-up.
Apart from learning how to make a web page you need to learn about file paths and how they are related to URLs and you need to learn some basics about web-servers --- at the very least.
There are also plenty of web sites that can instruct you in the basics of html etc. Macormedia's web site will have resources to help you as well as Dreamveaver's on help system and manuals.