If you are serious about learning DotNetNuke, DO NOT start with the source package. Instead, start with the INSTALL package. Do not mix this up with Visual Studio, not at this point.
Besides the documentation here, there are some very good installation guides. I'd recommend Mitchel Sellers' blog and also DNNCreative Magazine.
Basically, though, you just unzip the install package into a directory, make sure that the directory has the correct permissions (your asp.net user must have full permissions). Then, create a website with IIS and point it at the directory. Browse to the directory and follow the instructions.
You'll need to have asp.net version 2 or higher and AJAX installed for most of this to work, but that's about it.
Later ... if you are interested in developing modules, you can do that, too, without ever looking at a single line of the DotNetNuke source. You will, though, want to become very familiar with Michael Washington's adefwebserver.com site. You'll probably also want to get any or all of the Wrox books with DotNetNuke in the title. And don't forget to frequent these forums, read the blogs, etc.
You'll also want to start exploring the world of third-party modules for DotNetNuke if and when you need capabilities that aren't available from the core modules. Everyone has favorites, so start looking around.
And, as I said at the beginning ... put the source code away. Messing with it will only impede your progress.