Screen Shots

How to take a screenshot on your Mac - Apple Support

Take a screenshot of your whole screen

1. Press Command (⌘)-Shift-3.
2. Find the screenshot as a .png file on your desktop.

Take a screenshot of part of your screen

1. Press Command-Shift-4. The pointer changes to a crosshair pointer.
2. Move the crosshair pointer to where you want to start the screenshot.
3. Drag to select an area. While dragging, you can hold the Shift key, Option key, or Space bar to change the way the selection moves.
   
4. When you've selected the area you want, release your mouse or trackpad button. To cancel, press the Escape (esc) key before you release the button.
5. Find the screenshot as a .png file on your desktop.

You're doing nothing wrong... Those are the default keyboard shortcuts for screenshots. Keyboard shortcuts can be changed by going to System Preferences (under the Apple menu)->Keyboard->Shortcuts, look for Screen Shots in the list there. That will show you what your computer currently is using as the screen shot keyboard shortcuts. I can't imagine they'd be different from what you've seen in "how to" tutorials and are the defaults, but looking there will show you exactly what your Mac thinks the shortcuts are.