If you're doing it online, you want a PowerPoint presentation or equivalent. The associated materials they need is the sample character sheet you'll be using in your examples and a full set of dice.
Slide one should have a picture of the Character Sheet in the centre. It should then bring up arrows pointing to all the various sections with subsequent "Move Forward" presses. This shows them where to find things on the sheet.
Slide two shows the core mechanic (d20 + modifiers), then walks them through some simple checks. Do a skill check, an attribute check and a saving throw. Don't do an attack roll at this stage.
Slide three walks them through a very simple combat round. Start with an initiative roll. Then an attack roll/damage roll sequence for one side. Then the same for the other.
Slide four: talk about movement. Ignore AoO for the moment.
Slide five: action types.
Slide six-eight (nine?): Attacks of Opportunity.
I've found that the best order in which to explain these is as follows:
1) An attack of opportunity is a free attack caused when one side does something to drop their guard.
2) How to make an attack of opportunity.
3) When the AoO occurs - directly before the event that caused the AoO.
4) What you can do as an AoO (basically, anything you can do as an 'attack', so includes trip, sunder, etc. And yes, an AoO can provoke an AoO in turn.)
5) Threatened areas.
6) Actions that cause an AoO.
7) Movements that cause an AoO.
8) Consequences of an AoO (basically, you lose hit points, but generally continue as before)
9) Special cases: feats to negate AoOs, casting defensively, Grapples, etc.
I think that's it as the basic primer, although I'm sure I've missed some crucial elements.
You probably need an expansion to the presentation (or perhaps another presentation) that goes into character creation. In my experience, though, it's best to get them through the basics of the game, and have them play a session or two with pregen characters, and then hit them with chargen.
You then have a more advanced presentation that goes into things like DR, SR and other effects, but it's generally best to introduce these things gradually as they crop up.
At some point, you need to point them to the PHB, though, and have them read it.
Oh, and a thought occurs: why don't Wizards have this sort of thing on their website?
