Machan you can do it by yourself.
First download the following.
1. iOS firmware - iOS 4.2.1 for 3GS (
http://appldnld.apple.com/iPhone4/061-9895.20101122.Cdew2/iPhone2,1_4.2.1_8C148a_Restore.ipsw)
2.Sn0wbreeze - (
http://appldnld.apple.com/iPhone4/061-9895.20101122.Cdew2/iPhone2,1_4.2.1_8C148a_Restore.ipsw)
3. itunes 10.1 or higher.
Steps to restore iOS.
1. First go to C:\widows\system32\drivers\etc.
2. You will find a file named "hosts". Open it with notepad and add "74.208.10.249 gs.apple.com" at the bottom.
3. Now click save. It will say that you can't overight the document. So save the document somewhere else. Now delete the extension of the file and copy it to the C:\widows\system32\drivers\etc. It will overight the existing "hosts" file.
4. Open sn0wbreeze. (If it gives an error, download java form here -
www.java.com)
5. After you open it, Simply follow the instructions (At some point they'll ask to select a mode, you have to select "Simple Mode)" and select the ispw you downloaded earlier(It takes some time. So be patient) This will create a custom iOS. It will be saved to the desktop.
6. Open itunes. Connect the phone and put it on DFU mode. When your phone is in DFU mode, screen will be black but still PC recognizes the phone. If the phone shows itunes and data cable, then it is not on DFU mode. So make sure that your phone is in DFU mode.
7. After PC recognizes the phone it will pop up a message saying that your phone is in recovery mode. Now select the option "restore" on your itunes while pressing left shift button.
8. A new window will open. Select the customized ispw (The one that sn0wbreeze made, Not you one you downloaded).
9. After selecting the ispw, the new iOS will be installed. When it completes phone will restart and it will be already jailbroken.
10. If your phone is not a factory unlocked, remove your sim, launch cydia, then manage>souces>edit>add. Then type the following:
http://repo666.ultrasn0w.com
11. Then download the above software and install it.
12. Restart the phone. Insert the SIM. Put it on airplane mode. Again put it on normal mode. Your phone is UNLOCKED.