jailbreak it first:
first, update itunes to 10.2.2 if its an older version, use updated itunes to update your phone to 4.3.3 firmware if it's an older firmware.
then go to http://www.justmobilephone.com/download-redsnow/ and download appropriate version of redsnow for your pc/mac. then go to http://www.iphoneheat.com/2011/03/download-ios-4-3-1-firmware-for-iphone-ipad-ipod-touch-ipsw/ and download the firmware on which your iphone is currently running. if it's not listed there, find it through google search and download it. move it to desktop. start redsnow you downloaded earlier and follow the steps. firstly, it'll ask to choose IPSW, which is the firmware you downloaded. select it and let it work. then you'll have to put your phone in DFU mode, instructions will be shown on the redsnow window but it'll take a few times to get it right. after that, your job is done, everything will happen on its own.
to unlock: install a software like ziphone (there a more, you can find them through google) and let the magic happen