Command+Tab: By using these two buttons it let's you cycle through the different apps you have opened.
Command+Spacebar: This shortcut will open spotlight in the top right corner of the screen. In spotlight you can search anything on your computer. So it's really quick to lunch the spotlight by using this keyboard shortcut.
Command+Q: This shortcut helps you to quit programs. So if you're in firefox and you're done using it and you want to quit, you can use Command and then Q...