We frequently get people here who think they can design their own themes. this is not possible here but it is possible if you hire a web host and set up a WordPress.ORG install.
A blog is a website. This is all semantics and blog structure. A blog is a post based website designed for interactive communication. As the posts are frequently updated and appear on the front page of the site it's very attractive to search engines.
What most people call a website is merely a page based site that functions as a one way noticeboard. As the structure is page based and as pages are for static content that rarely changes, and as pages do not have RSS feeds it is not as attractive to search engines as a post based blog structured site is. When it also has a static front page that compels visitors to click through a static front page every time they visit the site then the blogger has made a traffic killing choice.
Any WordPress blog can be restructured from being post based to being page based - no upgrade required. For greater clarity read > http://onecoolsitebloggingtips.com/2013/07/25/create-a-wordpress-website-step-by-step/
Then see here > http://en.support.wordpress.com/using-wordpress-to-create-a-website/
The only themes we can use on free hosted WordPress.com blogs are found here http://wordpress.com/themes/ There is no FTP access to free hosted WordPress.com blogs for security reasons and we cannot:
upload any third party themes,
create child themes,
create our own themes,
edit templates,
create own own layouts,
or use stylesheets from other themes.
http://en.support.wordpress.com/ftp-access/
http://en.support.wordpress.com/themes/adding-new-themes/
There is no upgrade you can purchase that changes that.
On one hand, none of the characteristic "blog" features like categories, tags, authors, dates, etc. can be removed from a free hosted WordPress.com blog, without purchasing an annually renewable custom design upgrade and doing the required CSS editing.
http://en.support.wordpress.com/custom-design/
http://en.support.wordpress.com/custom-design/#frequently-asked-questions
http://en.support.wordpress.com/custom-design/editing-css/
On the other hand, one can edit a WordPress.org theme free of charge. Read the comparison > WordPress.com and WordPress.org: The Differences http://support.wordpress.com/com-vs-org/