If it's on a laptop, hold down the Fn key and press Prnt Scrn in the top right hand of your keyboard.
If it's a desktop, there should be a key similar to Prnt Scrn in the top right portion of your keyboard.
Then open up a new picture, like a .bmp, and press Ctrl+V to paste it in.
heh, I've noticed you posting around, your pretty new to RMXP right?
Good luck in your endeavors