Denise Robinson is the first girlfriend that you in the game, as she automatically becomes one when you finish the "Burning Desire" mission. She is available between 16:00 and 6:00 on the in-game clock, and her house is located on the street just south of your initial house in Ganton, Los Santos. There are no pre-requisites for CJ's appearance, so you are free to wear anything you want and he can be fat, muscular or skinny.
As for dates, Denise isn't too picky about where to go. For a Dinner Date, you can bring her to any fast food restaurant or to Ten Green Bottles, a bar just northwest of her house; she hates diners, restaurants and Burger Shot. For a Dancing Date, you can bring her to Alhambra, a club in Idlewood, Los Santos (it's the only club in Los Santos). When she requests a Driving Date, she likes the area around her house and she doesn't really care about how fast or slow you go. Denise has a special date, where she requests you to drive through rival gang territories, so she can do drive-bys. If he asks for this and you do it, you will not fail the original date asked.
At around 40% relationship status, Denise will start asking you in for coffee after dates. Your rewards for dating Denise are access to her green Hustler (only available when she is home) at 50%, and she will give you a pimp suit at 100%. Note that you can push her Hustler into your garage anytime before you gain access to it, and it will be yours.