It's very handy to be able to take a snapshot of whatever is currently showing on your computer screen and it's an extremely easy trick to learn.
First, you need to find the printscreen key on your keyboard. Usually it's on the right side somewhere between the number pad and letters, above the arrow keys. It may say Print Screen or PrtScn or something similar. For an example, see the attached image.
Now that you've found it, press the key. Don't worry, it won't actually "print" what's on the screen. If you don't have and special, 3rd party screenshot software installed, hopefully nothing appeared to happen. What actually did happen though is Windows saved an image of exactly what the screen was showing at that time. It didn't save it to a file though, it saved it to the clipboard. That's the invisible place where things are stored when you copy them. Like if you were to copy some highlighted text, it goes to the clipboard. When you paste something, it pastes whatever is on the clipboard.
So logically, to view and save the screenshot, it needs to be pasted into a program that handles graphics. If you don't have a better graphics editor, every copy of windows has Microsoft Paint installed under Start > All Programs > Accessories > Paint. Once you have it open, all you need to do is click Paste on the edit menu or simply press ctrl and v. That will paste the stored screenshot into the program so you can save it.
If you want, you can select, re-copy, and paste only the part of the screenshot that you want or just leave it one big, unedited image. Then all you need to do is save it. Each graphics editing program has a slightly different saving menu but for Paint, you just go to File > save as and pick where to save it, what to name it, and what format to save it in. For the most realistic image in paint without too large of a file size, save it as a JPG.