If you see a thumbnail in the corner of your screen, you can click it to edit the screenshot.
Otherwise wait until the screenshot will be saved automatically to your “Desktop” folder as a PNG file.
Capturing using Keyboard Shortcuts
Whole display
Take a screenshot for the whole display:
Hold the keys Command + Shift + 3.
Capture a region
Take a screenshot for a region of the display:
1- Hold the keys Command + Shift + 4.
2- Drag the mouse cursor to draw a rectangle indicating the area of the screenshot image.
Capture a window
Capture a screenshot for a window or a menu:
1- Bring the window that you want to capture to the front.
2- Press and hold: Shift + Command + Space + 4
The pointer changes to a camera icon.
3- To cancel taking the screenshot, press the Esc (Escape) key.
Capturing using Mouse or Software
Press Shift + Command + 5 (or use Launchpad) to open Screenshot and display the tools.
Icons from left to right:
1- Capture the entire screen
2- Capture a window
3- Capture a portion of the screen
4- Record the entire screen
5- Record a portion of the screen
For more details, see: Mark up files on Mac – Apple Support
Notes
To find screenshot files, by default,they are saved to your Desktop folder as .png files