I’m sorry but there isn’t a free and easy way to change fonts. If you are expecting to be able to click a button and change fonts throughout your blog then it’s not that easily done. Any font changes to font families, colors and size in the blog title, tagline, post titles, page titles cannot be done without CSS editing.
There are 4 ways to change fonts on wordpress.com blogs.
(1) Changing font families, colors and size in text in posts or pages either post by post or page by page by coding into the HTML editor. http://en.support.wordpress.com/editors/font-size-color/
(2) Changing font colors in text in posts or pages either post by post or page by page using the #4 icon (Select text color – change the text color) in Row 2 the Visual editor. http://en.support.wordpress.com/visual-editor/#row-2
(3) Changing font families (not colors or sizes) using TypeKit Fonts. http://typekit.assistly.com/customer/portal/articles/6840-using-typekit-with-wordpress-com
(4) Changing all font families, colors and sizes throughout the blog by purchasing an annually renewable CSS editing upgrade and doing the work yourself. http://en.support.wordpress.com/custom-css/