As long as her 'Like' percentage is above 60%, you can keep using this special ability.
If it drops below 60%, you won't be able to use it until you've had sex with her again.
The other three girlfriends require you to contact them by using the in-game internet.
Carmen and Kiki each have a profile on and Alexandra on in the "Party girl seeks man for NSA fun and blog material"- topic.
After having sex with one of them for the first time, she'll call you to tell you she might be able to help you with specific things, might you need them (like filling your health by calling her).
For your first date, you should wear decent clothes and drive a decent vehicle, however each girlfriend has different preferences.
It's important to live up to them, because if your girlfriend doesn't have 60% 'Like' at the end of the first date, you won't be able to ask her number.
Three of which are contacted via in-game websites, the other two through playing storyline missions.
Niko meets his first girlfriend, Michelle, during the mission Three's a Crowd.