Download Google Earth from http://earth.google.com/
What Do I Need to Run Google Earth?
Google Earth 4(beta) is available for Windows 2000/XP, Mac OS 10.3.9 or higher, and Linux
There are three main versions, free, Plus, and Pro. Google Earth Plus is only available for Windows users and adds GPS support as well as enhanced drawing and sketching. Google Earth Pro 4 beta is available for Mac or Windows. It is designed for commercial users.
Most users wont require more than the free version.
The Google Earth Interface
If you pass over the upper right-hand corner of the globe, the small compass will turn into the larger navigation control. Click and drag the circle to turn the map. North on the compass will move accordingly. Click on the arrows to move left or right, or use the star in the middle as a joystick to move in any direction. The dial to the right controls zoom levels.
Terrain and 3D buildings
Two layers are useful for creating a more three dimensional globe. Terrain simulates the elevation levels, so when you tilt your view, you can see mountains and other terrain objects. The 3D Buildings layer lets you zoom through cities, such as San Francisco, and fly between buildings. Buildings are only available for a limited number of cities, and they are only available in gray, unshaded shapes (although there is additional textured building info available for download.)
Advanced users can also create and texture their own buildings with Sketchup.