If you're using Windows or Linux, press the "Print Screen" button on your keyboard to record the whole screen. It's usually just to the right of the F12 key at the top. If you're using a Mac, press Cmd+Shift+3.
If you just want to record a single window, you can press the Alt and Print Screen buttons at the same time. To do the same with a Mac, press Command-Control-Shift-4, then space, then click a window.
Once you've taken the screenshot, come back to Tech Support Guy. Click inside the text box where you'd usually enter your message. Right-click in that message box and choose Paste.