Try this:
1. Close iTunes.
2. Connect your iPhone to your computer and then launch iTunes.
3. Turn your iPhone off (hold down the power button at the top of the iPhone).
4. Then, hold down the sleep/wake button and home button together for EXACTLY 10 SECONDS, then release the sleep/wake button.
5. Continue to hold down the Home button until a message appears in iTunes telling you an iPhone in recover mode has been detected. When you are in DFU mode your iPhone screen will be completely black. Remember that if you see the restore logo, iTunes logo, or any message on the iPhone screen, you are not in DFU mode but standard Recovery Mode.
6. Hit the OK message that should popup to restore your phone.
Fair warning: Apple warns that if you jailbreak an iPhone, you may NEVER be able to install a stock iOS back on the phone. If the above doesn't work, you can try an Apple store. They may or may not help you...jailbreaking voids all warranty & support.
Good luck.