Customising folder icons

Select the icon (has to be a .icns file) you want to put on the folder, press +I, click on the little icon on top of the new window so it is framed blue, press +C.
Do the same thing with the folder you want to change. Instead of pressing +C, press +V to paste the icon.

I recommend this app for creating both .icns-files and folder icons.

 

Apart from what the other user told you, I'd suggest that if you're really into changing icons and the like, to download CandyBar. It lets you manage icons and assign them to generic folders, volumes, library folders, applications, etc.. To obtain pre-made icons I also suggest InterfaceLift to download icons among other visual upgrades for your Mac. Hope that helps.