Default described above removes the UI.
If you want a screenshot with UI, You can use a 3rd party program like free, small, lightweight, 'Screenhunter' to take a pic, but the trick is... you put FFXIV in windowed mode, AND, you leave a tiny bit of desktop showing, so you can click on the desktop, thus making desktop and screenhunter active. If FFXIV is the 'active' program for mouse clicks and such, Screenhunter won't respond to your keyboard. You click on the desktop sliver to make desktop , not FFXIV active, each and every time you want to activate Screenhunter for a pic.
Btw, a lot of people take pics with names still on. Awful SE programming not to have a key for toggling names, but if you want a 'clean' pick, go into options and turn names off too.