How do I take a picture of my screen so I can upload it to Imgur and then post it on reddit?

Oh I know I should have learned this years ago but for some reason I never have. The help tells me to press the "print screen" button but that does nothing when I do it on my computer.

Help an old man finally learn something he should have learned years ago...., please. :)

Edit: Bless you all. You've made an old man happy!