This is an area I am just getting into so I do not have a complete answer. Are you looking to replace the site name text and style it?
The location of the text will vary according to the theme you are using. If you do not have firebug, I suggest you get it. It is an add-on that works with firefox browser and is essential for figuring out where things are. If you right click on the site name you can "inspect element" to find out what region name and css class is styling it.
You can place whatever text you want in a block and define a css type for styling it.
You can also do it programmatically. The text will have to be inserted in the php code file page.php.tpl. If you are not familiar yet, the php.tpl files actually construct the html code for a page when the request for the page is made. There are others, besides page but odds are that is where the code is. The style of the text can be redefined using css files.