Ah ha! I can actually help someone here!
First off, are you on a mac, or a PC?
If you're on a PC:
press Capture screen ... Simple as that.
If you're on a PC without the Capture Screen button:
Hit escape, go to key bindings, and scroll down until you find the screenshot one, highlight it and set it to something new... can't remember what category it's in...
If you're on a Mac:
Press F13
If you're on a Mac without F13 (i.e. laptop)
Hit escape, go to key bindings, and scroll down until you find the screenshot one, highlight it and set it to something new... can't remember what category it's in...
Note: it should read "Screen Captured" if the screenshot was successful.