Click here for all the important links:
Be sure to check out State of Jailbreak for the latest on all things jailbreaking: http://stateofjailbreak.com/
Download redsn0w 0.9.6b5 for Windows and Mac: http://blog.iphone-dev.org/post/1718400992/ultra-recycle
Find Firmware Files (IPSW): http://stateofjailbreak.com/tutorials/find-iphone-firmware-files/
Not sure what device and model you have? http://stateofjailbreak.com/what-device-do-i-have/
Find out if you have a new or old bootrom iPhone 3GS: http://stateofjailbreak.com/tutorials/new-or-old-bootrom-iphone-3gs/
In this video I give a full tutorial on how do jailbreak your iPhone 3G, iPhone 3GS, iPhone 4, iPod touch 2G, iPod touch 3G, iPod touch 4G and iPad on iOS 4.2.1 using redsn0w 0.9.6b5.