First step to screen capturing, is to pick something you want to take a Print Screen of. (layout previews, certain comments, recent layouts, etc..)

look on your keyboard and find the Print Screen or Prt Scr key. (it should be located near top right of the keyboard)
mac users press : Apple-Shift-3, or Command-Shift-3

after you press that key, open a picturing editing software (MS Paint, Photoshop, etc..)

press Ctrl + V, or edit paste. crop or leave as. then save the file as a .PNG (so it will look nice), name the document, and upload it to photobucket or somewhere

hope this helps!