You can't highlight a language with Markdown.  You need to outsource this to external libraries.  Such as.

  1. [SyntaxHighlighter][1]
  2. [Google Code Prettify][2]
  3. [Pygments][3]

  [1]: http://alexgorbatchev.com/SyntaxHighlighter/
  [2]: http://code.google.com/p/google-code-prettify/
  [3]: http://pygments.org/

**EDIT:** The StackOverflow way of doing this is to have a comment at the top.  Like so.

    <!-- language: lang-js -->

Just replace `lang-js` with [one of the language codes][4] or a tag to set the language to the language associated with that tag.  For example.

    <!-- language: C# -->

Highlighting for C# is associated with the C# tag.

  [4]: https://meta.stackexchange.com/questions/63800/interface-options-for-specifying-language-prettify/81970#81970