Hundreds of scripts, for a wide variety of popular sites, are already available at http://userscripts.org.
Before to Install Grease Monkey you Need to install FireFox so...
Latest Forefox from Here....
Grease Monkey from Here....
Now What is GM Script..??
GMScript is Short of Grease Monkey Scripts a bunch of Javascrits witch make change in yout HTML and Browsers..(-: