Generally when you are looking at the folder structure of a craft install your template files will be in
craft/templates/ (from doc root)
This is where all of your site’s templates go.
You can get more info about Craft's folder structure from the docs here: http://buildwithcraft.com/docs/folder-structure
Craft has a standard template path format that applies to each of
these cases: a Unix-style file system path to the template file,
relative from your craft/templates folder.
If you have a template located at craft/templates/foo/bar.html, the
following template paths would point to it:
If you name your template “index.html”, you don’t need to specify it
in the template path.
For example, if you have a template located at
craft/templates/foo/bar/index.html, the following template paths would
point to it:
If you need more information about templating you can also find that in the docs here: http://buildwithcraft.com/docs/templating-overview
As for the CSS files etc, it would really depend on the developer who set the site up, but the first place I would look is inside the
public folder or whichever folder is considered the document root of the server.
Hope that helps get you started!