Do a search for the iPhone development team and from there you'll find links to software that will jailbreak your phone. Do not pay for the software! It is free for all to use. Let us know if you're still having difficulty. The software will create the modified OP system file you require. You are not downloading a IPSW. It is created after you run through certain parameters of the program. Just be cognizant of the proper file to run since it is dependent on the firmware your phone is currently operating under.
If so, download redsn0w (if you haven't already done so) and also have the 3.0 .ipsw file available as well (you may have to download also).
Run redsn0w and use the 3.0 .ipsw file to jailbreak your device.
If you have problems with DFU etc then there are guides on how to do so all over the web.
In short- hold both power and home buttons for 10 seconds.
Release the Power button on the 10th second. Then continue to hold the home button for a further 10 seconds then release. This should put your phone in DFU mode. Redsn0w will detect that your phone is ready to jailbreak and it will continue automatically. If it stalls, then pull your usb cable out of the phone and plug it back in quickly- it worked for me.
If you get a white screen when your re-plug your cable back into the phone, simply remove the cable and hold the power button until the phone switches off. You may have to put it in DFU again. I had massive problems the last time I jailbroke.
If your still having no joy at all. Then try various USB slots or even a different cable.
That link is the file you need for redsn0w. You not only need to make sure you are using the 3.0 IPSW file, you also need to make sure it's the one for the 3G.
Try again using the file I linked. First, update your phone to 3.0.1. Then use redsn0w to jailbreak using the 3.0 IPSW file (NOT the 3.0.1 IPSW file). redsn0w will not work with the 3.0.1 IPSW. Once you are done, you will still be running 3.0.1.