You could use "Dotfuscator Community Edition" - it comes by default in Visual Studio 2008 Professional. You can read about it at:
http://msdn.microsoft.com/en-us/library/ms227240%28VS.80%29.aspx
http://www.preemptive.com/dotfuscator.html
The "Professional" version of the product costs money but is better.
Do you really need your code obfuscated? Usually there is very little wrong with your application being decompiled, unless it is used for security purposes. If you are worried about people "stealing" your code, don't be; the vast majority of people looking at your code will be for learning purposes. Anyway, there is no totally effective obfuscation strategy for .NET - someone with enough skill will always be able to decompile/change you application.
