If you are running your blog on WordPress.com, you just include the following lines before your code.
[sourcecode language="csharp"]
//your code comes here
[/sourcecode]
WordPress.com gives us the facility to avoid worrying about code highlighting. If your code is in between these blocks then it will automatically render as per the language you specified. It can support many languages.
Some of them are..
html
javascript
java
javafx
matlab (keywords only)
objc
perl
php
If you have a self-hosted site, or one hosted on wordpress.org, you should should use the SyntaxHighlighter plugin: wordpress.org/plugins/syntaxhighlighter.
This is the plugin the WordPress team sourced to create this functionality on wordpress.com
For more information see this link