First, you need to open the terminal window. Click the search icon in the top right of your screen (shown as a magnifying glass), enter terminal and click on the only result in the "Applications" section. It should open Terminal.
Once Terminal is open, enter this text: say hello world. Then press Enter. The computer should say "hello world"! If you want to make it say something else, type say and then what you want to hear!
Another fun thing to enter in Terminal makes your desktop become a screensaver. Enter this text to do so: /System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background
To change your desktop back to normal, close the Terminal window.