Google SketchUp is primarily a modeling software that is supposed to design houses, bridges and other types of structures for use with Google Earth. But you can use Google SketchUp for much much more, since it is a very good design program with many possibilities. Google SketchUp is easy to use, and you can find lots of tutorials and demos on how to get yourself started using it.
You can also purchase a license for Google SketchUp Pro, another version that has a couple of features that the free edition doesn't have. However, being priced at $495 it is probably this free edition that will be the best choice for most people.