You get two girlfriends through missions. One early in the game, and the second later after you unlock the third city. There is one in Blueberry just above Las Santos. Another is in the second city near the country club entrance. She is standing by some bushes. There is another girl at the driveing school in the second city. The next one is north of the second city. She'll be talking to someone in front of the police station. The girls you have to find have blue arrows above their heads.