In main storyline, when you first enter the Wasteland level up until at least level 2. Instead of going to Megaton or the GNR radio station, go to Rivet City, and talk to Doctor Li. If this is done when you get to the GNR radio station part of the storyline, you will be able to skip that entire section. Note: Rivet City is located near the end of the map in the southwest region.