how do i force restart my iphone

Hi all,

Just wondering if anyone can tell me how to "force restart" my iphone.

I have recently had my iphone exhanged at a store and everytime I connect to itunes it asks for a pasword. however there is no password set on the phone and nor is there an option to put a password in even if there was one. I rang up the iphone ppl and all they told me is to check the website, but i cant seem to find anything.

all help is appreciated!!!!