Modern Warfare 3 is mere hours away now. PC users are getting dedicated server support, but just in case you're new to the whole "dedicated servers" thing, Sledgehammer Games has written up a fairly detailed tutorial. It covers the basics on how to make your server and set your rules.
The Sledgehammer Blog entry covers setting a recipe (match rules) in the Private Match Setup screen, though you can also add criteria with a text editor if you'd like. Once you've selected a match type, the Options screen will let you tweak the particulars. Once you've saved the recipe, you can add it to the Playlist with other recipes to determine rotation, and then set your server live. To simply browse dedicated servers, you'll need to enable it in the Options menu.
It all seems straight-forward enough, and Sledgehammer promises a more advanced tutorial coming soon. Sledgehammer is actually one of three developers that Activision pulled together for Modern Warfare 3's development; the other two being Raven Software and Infinity Ward.