Open your application switcher
Command + Tab
From there, you can navigate with the arrow keys.
Command + Shift + Option + Esc for three seconds
This bypasses that annoying dialogue box that asks you if you're sure every time.
Screenshot your entire screen
Command + Shift + 3
Not just a selected area.
Screenshot a specific application
Command + Shift + 4 + Space Bar
A camera will appear, and you simply move it to the right window and click.