This is all about personal feel and how much the girl likes you.
The relationship aspect of Grand Theft Auto IV is perhaps one of the most nebulous areas of this epic game.
However, that shouldn't daunt you from building as many relationships as you can, because they happen to provide some of the deeper and more rewarding experiences Grand Theft Auto IV has to offer.
Niko and his date should have a good time from the get-go, so bring your girl to a local eatery or other place of amusement and have an awesome time.
Usually, this will involve a burger at Burger Shot, or perhaps some beers and darts at a local Irish pub. Regardless of what it is, try to diversify each time you go out with a girl, and be especially mindful to listen to their feedback as you walk around and drive around at all times.
There are five friends in total, which become available at different times during the storyline.
Each friend offers a 'special ability' in return for maintaining a good friendship.
There are five friends and five girlfriends in total, and each character has a unique set of preferences.
Each friend (and three of the girlfriends) provide special unlockable abilities in return for maintaining good relationships with them.
But for the most part, it's a safe bet that if you dress in at least moderately nice clothing, the girl you're dating isn't going to be disappointed.