Q:

How do I create a Google Sitemap for my ASP.NET website? 

I'm using DotNetNuke, so the whole site is basically dynamically driven and I'm clueless to how to go about creating a Google sitemap for it.  
Any help will be greatly appreciated. 

A:

DotNetNuke out of the box will create it for you if you are working with DNN 4.5.x and later (I believe that was the version). 
Just point google to
http://www.yoursite.com/sitemap.aspx
For example here is my site map. 
It isn't perfect, but works quite well, you can use some other tools to create it as well, but you will need something that spiders the site to get that. 
Update
I thought I would expand a bit as to why it isn't perfect. 

All pages are given the same priority
It only grabs actual pages, no third party modules that generate "pages" will be captured and included (Blogs, Ventrian News Articles, Forums etc). 

Depending on your needs BiteTheBullet.co.uk has a nice free provider,  Ifinity also has a good one as well that includes some of the "missed" content.  

A:

Jeff Atwood has some good information on SiteMaps here.   You can also see more information on SiteMap formats here.