Are u on a Mac? If so, press the "Command","Shift" and "3" keys simultaneously to take a picture of your entire screen. To take a selective picture, press "Command", "Shift" and "4" and select the area u want to take a picture of. Your pictures will automatically be saved on your desktop.
Ok so on the top, boven of your keyboard right of the F12 button there should be a key that says "PRTSC SYSRQ". Press that then go to paint and do CTRL V, save and ta da! :)
u can also use special software for taking screenshots - Jet Screenshot, for example. And if u use Google Chrome I'd recommend Awesome Screenshot add-on for capturing whole web pages of their specific areas.