I have seen a lot of questions about how to use Odin to flash your SGS4G. I am a noob as well and had to search throughout the forums to get the answers so here it is, plain and simple:
1. Download the Stock + Root rom (.tar file) posted by Krylon360 (kudos) HERE, or a rom from anyone else that has one being built for the SGS4G (use caution, do not attempt to flash a rom for a different device!)
2. Install the SGS usb driver 32-bit or 64-bit
3. Download Odin 1.7 HERE and RUN IT FIRST (before you plug in your phone in step 5 - this allows Odin to find your phone)
4. Remove your memory and sim card (I did this because Odin failed to flash my phone when they were present)
5. Put your SGS4G into Download mode (Hold volume up + volume down then plug in your usb cable running from the compy) You should see a big triangle symbol
6. Wait for your computer to install all drivers (takes a minute)
7. In Odin click the PDA button and select the rom (.tar file)
8. Make certain that Re-Partition is not checked
9. Click the start button and let it do its magic (takes a couple minutes)
10. After it completes your phone will reboot into recovery mode
11. Select reboot system now with the power button (takes a couple minutes to establish the new system)
Like everyone has said before, we are not responsible if you brick your phone. This process worked flawlessly for me.
My thanks go out to the people that are working on the roms and recovery systems that make this all possible. You should thank them by donating or by clicking the thanks button.