Actually, I think the only thing a neophyte needs to know are:
1) DM is the final adjudicator- don't argue too much, and don't take it personally.
2) Whatever rules are essential for running the PC in front of them- initiative, adding bonuses, etc.- or at least where to find the relevant numbers on a character sheet.
assuming, of course, the DM is willing to do a bit of hand-holding. After all, in the first combat, the DM can say- "OK, you roll a die like this one here to see who goes first...then roll this die here- add your BAB (its on your sheet right over there...) and then..."
A lot of stuff you can pick up during the flow of play.
With D20, the real trick I've found is guiding them through PC generation. Unlike many other systems, the future of a particular PC really can be decided by the decisions made during PC design.