Summarize:

This brings up the on-screen screenshot controls at the bottom of the screen. If you're using High Sierra (10.13) or earlier, press ⇧ Shift+⌘ Cmd+3 to capture the screen instead. It's the first button on the on-screen screenshot controls and looks like a macOS desktop. This changes your mouse pointer to a small camera. If you want to capture only one portion of the screen, such as a menu or window, you can use one of the other options on the toolbar, such as:   Capture Selected Window:  The second icon, which looks like an application window, allows you to capture a single open window on the screen.  Capture Selected Portion:  It's the third icon, which looks like a square made of a dotted line, allows you to click and drag a square around the part of the screen you want to capture. You'll see a small thumbnail at the bottom-right corner that confirms the screenshot has been captured. The screenshot is now saved as an image file on your desktop.  If you choose the Capture Selected Window option, click inside the window you want to capture instead. If you chose Capture Selected Portion, click and drag the mouse to draw around the area of the screen you want to capture, and then click Capture in the on-screen screenshot toolbar. If you'd like to send the screenshot to your printer:  Double-click the screenshot on your desktop to open it. Its name should be something like Screen Shot date at time.png. Press ⌘ Cmd+P to open the print dialog box. Select your printer from the drop-down menu on the bottom-left corner of the window. Click Print.
Press ⇧ Shift+⌘ Cmd+5 on your keyboard. Click the Capture Entire Screen icon. Click the screen to capture the screenshot. Print the screenshot.