If you need to take a screenshot on your Mac, but you do not have a keyboard or just prefer to use the mouse, here's what to do.
If you want to take a screenshot on your Mac without using your keyboard, you must first focus on the Finder by clicking on its icon in the Dock.
In the Finder menu bar at the top of the screen, click "Go" and then select "Applications" from the list.
When the "Applications" folder opens, double-click the "Utilities" folder.
In the "Utilities" folder, double-click the "Screenshot" application icon to open it.
When Screenshot opens, you will see a small toolbar appear near the bottom of the screen. Using this toolbar, you can configure the Screenshot application to take a picture of the entire screen, the selected window, or a selected part of the screen. You can also click on the "Options" menu and set a timer, among other settings. You can change any of these settings using your mouse - no keyboard required.
Once you have set your settings as you wish, click the "Capture" button.
A screenshot will be saved to the desired location (the default storage location is desktop.). After you take the screenshot, the toolbar will disappear, so if you want to take another shot, you will need to open the Screenshot application again.
Another way to take a screenshot without a keyboard
You can also take a screenshot using the Preview app. Just launch the application using the Finder method "Go"> "Applications" or using the Launchpad. When Preview opens, click on the "File" menu and select "Take Screenshot". And the process ends here!
Source of information: howtogeek.com