Often called screenshot or screen capture.
It depends a bit on what kind of HTC phone you got, just what model since the method differs a little bit.
Following works on ALL HTC Android phones
Standard Android way
Press in home and volume buttons at the very same time, keep pressed until you hear a camera shutter sound. Will give you a notification where you can easily share that screenshot. A rather clunky way of doing it, but it works. If you want to take a screenshot of the lock screen, this is the only way since you don’t have a home button visible then.
Following works on most HTC phones with HTC Sense software
HTC way via buttons
Keep power button pressed shortly and tap home button.
HTC way via quick menu
Just swipe down from notification bar with far enough so you see the quick settings. Or swipe down notification bar and then tap the upper part it, this will bring down the quick settings. Then you just tap the screenshot button, you might have to edit quick menu first (the edit button on top) to have it visible. Just find it and drag it to where you want it. Then just swipe down and tap screenshot. Done. Gives you a notification as well.