You can use jailbreakme.com to jailbreak your iOS version, now depend on the baseband you are on before you can think about using ultrasnow from Cydia.
Before you can follow these instructions you must have a jailbroken iPhone and you must be on the 04.26.08, 05.11.07, 05.12.01, 05.13.04, 06.15.00, or 01.59 baseband(modem firmware).
My bad about the 01.59, that is for the iphone 4 only i dont know why I keep thinking of the ip4 for.