1. Find the "Print Screen" button on your keyboard and press it.
2. Open up the Paint program by following the steps in this next pic...
3. Click Edit, Then Paste as shown in this next pic.
Now you should have picture there of what was showing on your computer monitor. There is a little blue dot at 3 different parts of the pic (circled in Red on this next pic). You can use these to change the right and bottom edges of the picture.
There are also other tools to use...the best ones will pop up if you click on the word "Image" at the top.
There is also a Toolbar on the left hand side...and a color "choice" bar at the bottom.
When you are done, click on File then Save As...and decide where you want to save the pic to. I always save as a "JPG" type file (like this).