Follow on screen instructions once application started carefully!!!

Step 1)
Go to Click the Windows link.
Step 2)
When prompted save the downloaded file.
Step 3)
Double click the downloaded file then drag the greenpois0n.exe file to the desktop. Double click greenpois0n.exe to launch the application.
Step 4)
Turn off your iPhone then click the large Prepare to Jailbreak (DFU) button
Step 5)
You will now be instructed on how to place your iPhone into DFU mode (see other post for guide).
Step 6)
Once your iPhone is in DFU mode you may click the Jailbreak button.
Step 7)
Jailbreak should started on your device…
Step 8)
Click the Quit button when jailbreaking has been completed.
Step 9)
After the iPhone restarts find a new Loader application on your Springboard and press to launch it.
Step 10)
Press the Cydia icon then press Install Cydia button.
Step 11)
Once Cydia has installed successfully you can remove Loader from your device. Press the Remove button then press the Home button. Your device will now automatically reboot. When your iPhone has restarted run Cydia and let it do all the necessary updates.