>>1
Find out her online account information. If she has a Myspace or something similar, even better. You'll need it.

Depending on her age, take photos of her being intimate and post them secretly online. Include a blog entry or something about how easy she is, and describe what she says/does when having sex. Do not let anyone know about it just yet.

Get her to masturbate for you, and secretly record her moaning as an mp3 file. You'll be using that on the website with the pictures.

Secretly (of course) record the two of you fucking. On her parents bed. Make sure she calls you "Daddy" often. Also make sure the camera is set so you aren't shown/aren't shown well.

Invite her over to your house at a specific time. Tell her to wear something sexy. When she arrives, be in the process of cuddling with some other girl (preferably the easiest girl you know). Invite her to join in. If she screams, good. If she joins in, even better. If she does join in (not likely, but if you word it right you might perk her interest), also call some of your male friends over to join in--make it an orgy. Record this without anyone knowing.

Upload everything to the internets using her information/her myspace/whatever. Be shocked and appauled the next day after word has spread about your girlfriend the whore.