You can't highlight a language with Markdown. You need to outsource this to external libraries. Such as.
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 or a tag to set the language to the language associated with that tag. For example.
<!-- language: C#c# -->
Highlighting for C#c# is associated with the C#c# tag.
(use lowercase letters, uppercase letters like <!-- language: C# -->
does not work.)