Unlocking an iPhone allows one to use a mobile phone service provider of their choice and the steps to get it done is simple to follow but vary from one brand to the other, however there are general steps to follow on how to unlock your iPhone 3g first upgrade the device with the latest firmware by popping up iTunes and update by clicking the update button to update the device with the latest updates and it is important to backup or sync the device first before performing any of the task to ensure no data is lost.
Download quickpwn an application that will help in jail breaking the device before unlocking, connect the device to a computer using USB and launch the application. Select the device firmware available, install cydia or installer and follow the directions as you wait for it to load. Install repositories on the device using the earlier application then use cydia to install yellow snow once the server allows you to do so and run the application. Turn off the device and remove the SIM card from the device and insert one from a desired service provider and turn it back on, it should pop up after a short while if not one should repeat the process.