Do you have a WordPress.com blog? If so, please provide a link.
In WordPress.com, you can change the fonts with CSS. But you have to buy the upgrade, and you should know how to write CSS because there are only a few people here who can help you with it. You can practice without purchasing the upgrade. http://support.wordpress.com/editing-css/
That said, you can change color, size, and font in individual posts with HTML.