When Unity first opens, you should see a splash screen referring you to different tutorial resources and language guides. How helpful! Now close it. (Don't worry, it'll be there next time, unless you uncheck the Show at Startup box).

Because you chose to download the sample project, it should automatically load the first time you run Unity. Navigate to Window | Layouts | 2 by 3 to see the different panels that we're about to tour.

To try out the demo, click on the Play button at the top-center of the screen.

You can walk around the Island Demo using the WASD keys on your keyboard. Jump by pressing the Space bar. When you're finished exploring, click on the Play button again to end the demo.