The following steps are for the iPhone 3G only using an Windows PC.  

The following instructions will jailbreak all iPhone 3G phones however you will not be able to unlock your iPhone if it has modem firmware version 05.14.02 or 05.15.04 you can confirm your modem firmware on your iPhone under settings->general->about

Download the following file using the Firefox Browser which can be downloaded fromwww.firefox.com

Download File 2: Apple Firmware used for preparing for the jailbreak

Click here to download 

Note:  Save the File do NOT select open in iTunes

Please now select one of the following options:

Option 1: Click here if you use your iPhone on its original network

Option 2: Click here if you do NOT use your iPhone on its original network.