Go to this
link and download the 3.0.1 (3G): iPhone1,2_3.0.1_7A400_Restore.ipsw and 3.0.0 (3G): iPhone1,2_3.0_7A341_Restore.ipsw firmware to your desktop. Create a Folder where to put all your files, then download RedSn0w 8 from
here and put it in the same folder. Also be sure to have latest itunes version installed.
First, open iTunes and connect your iphone to it, if you are currently using your iPhone and have data on it, just right click your iPhone on Itunes and do a backup first, then once it finishes just press Shift and Click the restore option on Itunes (obviously while you are in the iPhones area) it will ask you for a "Backup file" to restore, just select the 3.0.1 (3G): iPhone1,2_3.0.1_7A400_Restore.ipsw you saved earlier, let Itunes restore to that firmware, it will erase everything and will put your iPhones as a brand new phone with latest jailbroken and Unlockable firmware version, 3.0.1.
Once you finish, then follow
this instructions on how to jailbreak your iphone and you will be done. remember, when jailbreaking, use the iPhone1,2_3.0_7A341_Restore.ipsw firmware (that's why I told you to download both firmware versions) and not the 3.0.1, just be sure and you won't have any issues.