To make screenshot, simply simultaneously press the power key and the volume down key:
and you're done!
To view the image, just display the notification area, sliding your finger from the top down. In the notification area message "Screenshot captured" should appear. Clicking on the message, we can open the image with the "Album" application, and from there we can send it using any of the applications you have installed: Facebook, Twitter, MMS, Gmail, ...)