I am a veteran newb-trainer. I've designed my own system to help this, but it's d20 based (almost ready for the public), and so far it's loads of fun. However, most training has been done with D&D/PF
1) Rules light is your friend. Just ask them WHO they are, and describe a scenario. They will fill in the blanks. They don't need rules yet. What rules you'll want are
ABILITY SCORES: the shape of the character.
HIT POINTS: enough that she can fall out of a tree and not end the game.
DEFENCES: Do 10+modifiers, do not do THAC0 or inverted anything. They're great for a later game, but the community has been doing fine without them, and they screw up the math later on. IMHO.
2) Your girlfriend, however, is likely smart enough to dominate the rules. Really, while 99% of newbs I'd start on the game sans rules, you could be romantic and get her a copy of the PHB or three. She's got a custom vampire MTG deck, so the only reason you'd hold back the rules is because you FEAR HER WRATH (mwah-hahahahahhaaaaa). Don't be a wimp, suck it up, and unleash her inner power-gamer upon the world.
3) The difference between rpgs and the games she knows, however, is that RPGs have the strength of going outside the box. You can get away with not having rules, and she might even have more fun.
Here's an example scenario I like to use:
The PCs start in a village. They hear a cat has gone up a tree and a little girl asks them to help get it down. They figure out a way to get up there, convince the cat to get down/grab it, and get down with the cat. You can even have them see a glint of silver and find (gasp!) a magic dagger in the tree with a fiery glowing gem in the hilt (plot device and instant treasure to get them hooked).
The PCs then hear that a woman has dropped something down a well (i used a healing potion for a very sick man). If they climb down, they discover the entrance to a dungeon. If they explore they find it has a set of old tests for a now long-dead adventuring company that will get them up a level. Lots of skill tests in the dungeon, with one or two combats with stray creatures, and some treasure (a couple dead adventurers from ages ago, their gear still on them, plus a map to the next adventure site)
Before they can just jump in, however, feel free to have them go about town, talking to NPCs and buying their equipment. That will cut down on character creation: they make their ability scores, pick a class, put clothes on their back and shoes on their feet, and give them a bag of gold to suit up. After that, the adventure will write itself.
Make sure to make her/newbs the hero: don't let a GM-pc do all the work. You want them feeling empowered, or else you're ruining the fun. Reward creative thinking: it's an investment to future game sessions.