The method for taking a screenshot depends on the device and operating system you're using:
Computer
Mac (Apple) :
There are multiple options, with the simplest being: SHIFT + COMMAND + 3 to capture your entire screen.
Screenshots will be saved to your desktop.
Windows:
Many options are available, with the simplest being: Press WINDOWS + PRINT SCREEN simultaneously to capture your entire screen.
Screenshots can be found in Pictures, Screenshots.
Mobile device
iPhones or iPads with a home button:
Simultaneously press the Home and Side buttons, then release.
iPhones or iPads without a home button:
Simultaneously press the Side and Volume up buttons, then release.
Android devices:
Press the Power and Volume down buttons simultaneously; the screen should flash briefly, indicating a screenshot has been taken.
Alternatively, press and hold the Power button to display several options, and select Screenshot.
If you encounter difficulties with any of the methods above, search for your device's name and model number online to find numerous guides that can assist you.