In order to jailbreak and unlock an IPhone 3 G with redsnow v 0.8, you will need to use Windows Vista. When it asks if you are sure you want to run this software, then you should click "run".
Browse your files and search for "iPhone 2.1 3.0_7A341_Restore". Click on it, and it should show that it is processing. Click "next". Install Cydia, by clicking "next". Next, turn off your iPhone and disconnect it from the computer. The computer will advise you to do this. It will take at least 5 seconds to completely power down.
Hold down "home", while you plug the USB cable into the iPhone. Depress the home key until logos appear.