Jailbreaking an iPhone is pretty easy due to the massive popularity of the device.
In this Tutorial, i will cover JailBreaking iOS 4.2.1 "untethered" with Greenpois0n.
This has been tested with:
My personal iPhone 4 running iOS 4.2.1 not on the AT&T network
Required:
Greenpois0n
A PC or Mac
An iDevice with iOS 4.2.1
...
JailBreak an iOS 4.2.1 Device