The Situation: you have a million screenshots on your desktop. You curse mess and want to take control. Great! Set a folder to keep all the screenshots automatically, rather than turning your desktop into unwieldy monster.
make a folder on your mac where you want to save your screenshots. Name it whatever you want.
open the terminal.
paste in this command: defaults write com.apple.screencapture location
don't hit enter yet. after the the word "location" enter a space then drag and drop the new folder you created for the screenshots into the terminal window. That will auto fill the path to the folder, and should look something like this (if the folder is on the desktop titled "Screenshots"):