how do i restore my iphone 4?

hi everybody, i bought a iphone 4 on thursday and its broken i hope somebody can help me, all i get on the screen is the apple logo and when i connect to itunes (latest version) it states iphone is in recovery mode and needs to be restored, so i try to restore the phone and get so far and then a error message appears saying the iphone could not be restored. An unknown error occurred (21) and thats as far as i get 😟 any help would be greatly appreciated as i've got no phone for work now and wont be able to make it to a genius bar for a couple of days due to work commitments pls help....