I have a car, it has an issue, can you fix it?
thats a very big question..
if you have a look at http://codex.wordpress.org/Stepping_Into_Templates you will see that parts of the site are located in different sections.. ie headers and footers that are common across the site have their own files..
however, the simpler answer is the HTML that a user sees in the ‘view source’ section is created by the files in your template folder.. located in
site_root\wp-content\themes\theme_name
i wouldnt suggest touching much in the way of back end coding if you’re unfamiliar with it though, as its a good way to break things 🙂