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.
As you progress through the story, Niko will be able to wine and dine with a number of Liberty City women.
The other three girlfriends require you to contact them by using the in-game internet.
Dating is much like hanging out with friends in that each girl has their own preferable activities.
Conversely though, Niko's choice of wardrobe and vehicular transport will also affect whether or not his date has a good time.
It's been a while, and she's still sending those text messages asking to take her out on a date.
As in GTA San Andreas, GTA IV features the possibility to take girls out on a date.