From the Tails UI, select Applications > Utilities > Screenshot

This will open up the Screenshot utility. Form here you can:

Take a screenshot of the entire desktop.
Take a screenshot of the current window only.
Select a specific area to take a screenshot of.

You can also change settings, such as:

Time-delay your screenshot.
Include your pointer.
Include the window border
Apply effects.

Once you have taken a screenshot, you can name it and save it wherever you wish - including a USB if you wish to keep it off-system, perhaps with your USB Tails bootloader.

You can also use the Print Screen button on your keyboard to take a screenshot:

PrtScr (Prt Sc on some devices) will take a screenshot of your entire UX screen.
Alt + PrtScr will take a screenshot of your current active window.
Your screenshot will be saved to Home by default.