I have added an article now I want to know how I change font and font size of that article.
You usually do this through css. You'd target the header mark-up (usually H2) and set the font in the css your theme is using.
But there is also a great module that allows you to access tons of free fonts (and some paid fonts) on the web. You can assign this custom font to some or all specific mark up regions in your page.
A list of some of the Drupal sites I have designed and/or developed can be viewed at http://motioncity.com/drupal
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