Find "snipping tool" and create a shortcut to it.
Usually it's called "snippingtool.exe" and in your Windows directory. If it's in your PATH you can just create a shortcut without looking anything up. To do this right on any folder or on your desktop, navigate to "new" -> "short cut". In the box that says Type the name of the location:, enter "snippingtool" without the space, hit next and finish.
Snipping tool lets you select any area of your screen to capture.
Like Cat Harrison said, you can use the "Print Screen" key on your keyboard. It's normally found near the "Insert" or "Pause/Break" key. Hit this once to copy the entire screen to your clipboard. paste this into your favorite editor (mspaint does fine) and crop it to the proper area. You may also use "Alt+Print Screen" to capture the active window into your clipboard. Again, paste this into your favorite image editor and crop if necessary.