you have to have your browser on fullscreen, then press the prnt screen button on your keyboard.
1. Press the Print Screen key on your keyboard.

2. Open an image editing program, such as Microsoft Paint. If you dont know where that is, click Start... Run... and type MSPAINT and then click OK.
3. Then press Control-V to paste the screenshot into Paint
4. Then select File... Save... select PNG as the Filetype if you want it to look better, and type the name for your screenshot.