This one needs no introduction, and I'm going to assume you've already got SketchUp installed on your system because installation is straightforward. Instead, I'll use the space allocated to SketchUp installation telling you about the extra texture pack you can have that will enhance SketchUp even more.
Here's how to find and install the extra textures provided by Google.
Once you have SketchUp or SketchUp Pro installed, go to http://sketchup.google.com/intl/en/download/bonuspacks6.html and click on the download link for Windows or Mac. Run the file and follow the instructions.
When you're asked where to install ...