Facebook Share Command in Blog / Web Site

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I often visit a blog or site and found a facebook share link ( have facebook icon ) in a some site that i visit. It usually locate below the title part, or some time below the articles. The function of this thing is to make user that press it automatically share the blog link in his facebook. Now what i want to know is, how to create that link. What i need to do with my wordpress blog. Should i add a plugin or edit my theme. Any one can give me solution?