Your girlfriend's house is marked on the map by the heart.
As soon as the girl is picked up, you can, of course, take her on a date.
Then you can give her a lift home and when you are further into your relationship with her, she can invite you for a coffee.
You can choose from several gifts, that can make your girlfriend happy. When you choose the gift, use it like a gun, go up to your girlfriend and press TAB.
Dating any of the six girlfriends is not required for 100% Completion, with the exception of Millie Perkins (although Millie can be killed during her first date).
Denise is first encountered during the mission Burning Desire, whilst Millie first appears during Key To Her Heart.
Each girlfriend has their own different tastes to the three activities, as well as Carl's physical appearance.
Michelle Cannes, Barbara Schternvart, Helena Wankstein and Katie Zhan are the rest of the girlfriends, although they are encountered at specific locations around San Andreas and aren't tied to any missions.
If the girl is possible to pick up, you'll notice a blue arrow above her.
That is enough to go up to her and you'll either get her telephone number or you'll hear that you aren't right for her.
There are six possible girlfriends for CJ to date during San Andreas, although none of them are required to get 100%, with one small exception.
Two of the girlfriends are encounted during missions, while the rest are completely optional.