Instructions
Complete the Tenpenny mission "Burning Desire" early in the game, and you will meet Denise. As soon as the mission is over, a heart icon will appear over her house on the game map. Go to her house and initiate a romance side-mission at any point.
Drive to Denise's house and wait for her to get in. Your date will then begin.
Drive around a bit and listen to what Denise says. There will be a number of icons listed on your map for things such as restaurants and bars. When Denise gives an indication of the kind of place she'd like to go, drive there.
Get out of the car at your date location and, if prompted, kiss her.
Take her home to successfully complete the date and move up in her favor.
Return often to take Denise on more dates. If you wait too long, she'll call you and complain that you're ignoring her. If this happens, get over there as quickly as possible to jump-start her affections. If you fail to act, you will lose her as your girlfriend.