Tiffany is one of the four marriable characters. The other three are Kate (Intelligence), Victoria (Money), and Devin (Negative Karma).
Tiffany's dating route requires progressing amounts of strength in order to continue her questline. You can initially find her standing near Skye's Bar, across the street from Dominick. With the proper strength she will ask you to give a puppy to her sister, who is outside of the Penthouse building, near the bank. Tiffany's sister will thank you and ask you to deliver keys to Tiffany. Upon giving the keys to Tiffany she will gratefully give you her scarf and invite you to meet her at the club. Once it is night you can head to the club and find her next to the dance floor. She will comment on how cold it is and you can give her back her scarf. She will ask you to hold her purse for her while she dances. The following day, you can find her outside of the Mobster Tower, and give her back her purse.In exchange for the purse she will give you a watch that can be given to the jewel dealer in the Casino. He will give you a wedding ring. Once you obtain the ring you can go to Skye's Bar during the evening and ask her to marry you.
After marrying Tiffany you will get one hundred charm, and one charm a day everyday until you stop playing.