Making a screen capture is quite simple.

  1. When you have what you want to show on the screen, press and hold the CTRL button and press the "Print Screen" button. 

    Some keyboards might have different marking on the key like “PrtScn”, “PrtSc” or just "Print".
    Doing this will not save the image of the screen, it will makes a copy in the screen in your Windows clipboard.

  2. Once you have made a capture of the screen, don't panic, it it's invisible in your clipboard 
    Position your cursor in the ticket where you would like to insert the image.
    Press and hold Ctrl + V  to paste the image from the clipboard into your ticket.

You can also send an image file in a ticket,  by using the "Attach files" button 

Specific tools 
There are many specific tools to make advanced screen capture easier.

One of the best available here is FastStone capture