Call your friend and perform activities with him on a regular basis to increase your 'like stat'.
This stat (amongst other friend stats) can be accessed from the pause menu under the 'Stats' section.
Like the other relationships that Niko has with his friends in GTA IV, favor can be gained with girlfriends by taking them to different venues around the city.
Unlike the male friends, however, dating is more than just worrying about the venue.
The ability to date girlfriends, introduced in Grand Theft Auto: San Andreas, returns in Grand Theft Auto IV.