There are two females in the game you must date—Mallorie's 'friend' Michelle and the O'Reary boys' sister Kate—but other than that, dating is completely optional.
However, there's a lot of fun to be had by partaking in the Liberty City singles scene.
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.
It's best to wait until you have completed the main storyline before taking on the girls from the internet, since the more girlfriends you have, the more attention they require and the more often they will call.
Dating multiple girls at once is a risk if one of those girls is Kiki Jenkins, because she starts to stalk you, and eventually dumps you if she catches you with another woman.
If your friend calls but you aren't able to hang out, respond positively and then call immediatley afterwards to cancel the activity. There are five different girls which Niko can date, two of which are unlocked automatically during the storyline, and the rest of which are available via the in-game internet.
Call one of the girls to arrange a date, then pick her up within 1 hour (2 real-time minutes) and take her to one of the activities shown on the map.
You're going to have to go to an internet café and browse the in-game websites Love Meet and Craplist (which are always on your homepage when you logon).
Try to press the various girls on the site for dates, but keep in mind that there are some girls that you can date and others that won't date you no matter how hard you try.
So now that you know how you can potentially score a date with one of Liberty City's beautiful single females, what should you do when you're on a date?
Each friend offers a 'special ability' in return for maintaining a good friendship.
In order to unlock a friend's special ability, you must have a high 'like stat' for that friend, and this stat is based on how regularly you hang out with your friend.