The easiest way is to set your Visual Studio IDE to identify the changes you need to make.
You can do this in Visual Studio 2008 by going to:
Tools, Options, Text Editor, HTML, Validation and choosing the appropriate target.
Possibly XHTML 1.1 or XHTML 1.0 Transitional.
For some information on the different types, read:
http://msdn.microsoft.com/en-us/library/aa479043.aspx
Then you need to work through the points highlighted on your page.
