WordPress.com offers a really simple method to add a retweet button to your posts. This will enable your blog visitors to share your posts on Twitter. Simply add the following text to your post (visual or HTML tab, it doesn’t matter), wherever you want the button to appear.
[tweetmeme source= "<Your Twitter username>" only_single=false]
Replace the text in red with your own username but keep the quotation marks. By default, the button will be right-justified.