Download the Syntax Highlighter and upload the libraries in some web space. Link those libraries into the HTML code of your blog. Or you can also use the online version where the libraries are already present online. The demo below uses the online version.
I was looking for some support on how to use it, and ended up with something funny like this picture here. Well, they were using Syntax Highlighter to show 'how to use Syntax Highlighter'.
Anyways visit the site, download codes or use the online version.