You should have a "PrtScrn" button on your keyboard? Usually it's somewhere at the top right corner, above or near your Backspace key. Once you press it, the screenshot is in your clipboard, and you can simply create a new blank document in Photoshop, hit Ctrl V to paste the screenshot onto it, then save it as a jpeg to upload here.
Or, you can simply search in your Start Menu for "Snipping Tool". It's a great little app, very easy to use.