Ok, here's how you do it:
In game, press the Print Screen button (on keyboards it's either PrtScrn or PrintScrn, or PrntScrn).
Load Paint (it's in Start -> Programs -> Accessories -> Paint).
Hold down the CTRL button and the V button. The screenshot should appear.
Save the file.
If you're going to submit it to a site be sure to save it in .jpg format, because that's what nearly all sites accept screenshots as.
Hope this helps