Most keyboards will have a button, usually in the upper right corner area, that says Prnt Scrn or something very similar. That's your print screen button, and its what you'll press to capture an image of your screen.
From there, you'll need a program like Paint, Paintshop Pro, or a Photo Editor to paste it into. You'll want to open one of those programs, start a new file/sheet and right click anywhere on the blank page and select Paste from the drop-down menu.
You can edit your image inside these programs to show only want you want to show. Use the Select function to draw a dotted box around the portion you want to keep, and Image/Crop to remove the rest.
Do a file/save as and save your file somewhere on your computer.