You cannot "modify a template" as we cannot edit templates or themes on free hosted WordPress.com blogs. CSS editing can only be used to change appearance. It cannot be used to change the way any theme functions. Please read > Custom Design - FAQs
You can create a page based website structure though very easily, provided you comprehend the differences between posts and pages. Pages are static and sit outside of the blog structure and you cannot put posts on "pages." All posts will show on the main blog page. There is no way to exclude posts from the main posts (blog) page. See this support document on the differences between pages and posts.
At wordpress.com, you organize posts by categorizing them and then you can create a custom menu and put the categories into the custom menu.
If you do not want all the posts to show on the main page, then you can create a static front page for your site. If you wish, you can also create a "blog" page, but you do not have to if you do not want to.
Here’s a link to a custom menu walkthrough > http://onecoolsitebloggingtips.com/2011/08/11/wordpress-com-custom-menu-walk-through/
There are many common errors, misunderstanding and misconceptions when creating custom menus and you can read about them here > http://wpbtips.wordpress.com/2011/12/19/custom-menus/