how to wipe an iphone 3g

Follow the "Erase all content and settings" advice in this article http://support.apple.com/kb/HT3828
and all info will be deleted.

Do the "Erase all content and settings" as suggested, but after that I would also do a DFU Restore.

How to DFU Restore.
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1.) Open iTunes and connect the iPhone.
2.) Press and hold the Home button and the Sleep/Wake button at the same time.
3.) After the screen turns off, let go of the Sleep/Wake button and continue holding the Home button.
4.) iTunes will pop up a message telling you that it has detected an iPhone in recovery mode.
5.) Click restore to restore your phone to Factory settings.

Error 6 is most often caused by security software on your computer interfering with the restore process. So, disable or un-install all anti-virus, ant-malware and firewalls on your system. Then, do the following:

First, reset your phone:

Press the sleep/wake button & home button at the same time, keep pressing until you see the Apple logo, then release(ignore the slide to power off).

Then, see if you can force the phone into recovery mode:

Leave the USB cable connected to your computer, but NOT your phone, itunes running, press & hold the home button while connecting the USB cable to your dock connector, continue holding the home button until you see “Connect to iTunes” on the screen. You may now release the home button. iTunes should now display that it has detected your phone in recovery mode, if not quit and reopen iTunes. If you still don’t see the recovery message repeat these steps again. iTunes will give you the option to restore from a backup or set up as new.

Make an appointment at an Apple store, see if the geniuses can restore your phone. They will be able to confirm a hardware failure. There is no charge for this service.

There's also a program that forcibly kicks your phone out of recovery mode. I used it on my iPhone 3G when I downgraded back to 3.1.3.

It's extremely easy to use on a Mac, but on Windows it has some dependencys (you have to install .Net Framework 4.0 and libusb).

It's called RecBoot: http://joshuabailey1997.wordpress.com/2010/09/02/recboot-v1-3/

If that doesn't work just bring your phone to the genius bar, and have them restore it(like the previous poster said).