If you met her though the internet, you can re-contact her a few days later, via the same website on which you met her the first time.
If you finish a date succesfully with a girl who already liked you 100%, she'll send you a phone message with a photo of her.
To meet one of the girls, press Contact Me on the bottom of her respective page, and she will send you an e-mail some time during the following days.
If it says she's interested, upon reading the mail you'll get an update in your Organizer, featuring the time and date of your appointment.
There are more vehicles with which you can pick up your girlfriend stylishly.
In GTA IV & GTA TBOGT, you have the option to date fifteen women in Liberty City. This page contains spoilers, or hints about the game's storyline or progression.