Go back to 'Tw@' and check your mail. If it's been a few hours since you requested the date you should have a mail from Tom proposing a date at the 69th Street diner at a random time. Reply positively to this mail to confirm the date. You may have a lot of time before the date, if so you could go back to doing a more missions, or just save your game at the safehouse to advance the game 6 hours. You'll get a reminder two hours before the date and a heart icon will appear on the map showing you the location. You must get to the diner within 3 hours of the scheduled time for the date.
Next: Random Character: Mel