When it comes to rendering software, there are many options available nowadays, but Kerkythea is one of the oldest free software packages that is still available for this task. It isn't as easy to use as other rendering software, but it still works well and produces good results. Check out this video for an introduction.
This is a free sample instructional video from the book "Architectural Design with SketchUp: 3D Modeling, Extensions, BIM, Rendering, Making, and Scripting" (2nd Edition). I discuss this topic more in detail in that book's Chapter 2.Want more of these videos? If you own the book, use the password from the inside cover to gain access to all of my remaining videos on Wiley's website: http://www.wiley.com/go/schreyer2e. If you don't yet have your own copy, follow the links in the sidebar to get one. It is a great reference for SketchUp!