This is so easy when I learned it I felt like a JackA$$ for not know,

First pull up the screen you want to capture, next press the key "PrintScrn/SysRq" between F12 & Scroll Lock, then you just go to paint or any othe photo software and click Ctrl + v, there you have it!

This is perfect for proving stats and things like that!

Don't forget to add some Rep for me, if you don't know how, all you have to do is click the Little scale at the bottom of the table my name is in and click Approve or Disapprove, throw in a comment, click the botton and done!