You need to complete Cassie's quest. After that, Arlene will appear in the salon. Go to her and ask to become her submissive - she will say that you need to get permission from Fawn. Talk to Fawn and ask for a more hardcore session. Then, I think you can figure out what to do yourself.
Well, since you've reached Fawn, you must be familiar with Cassie - otherwise you wouldn't have been able to get to Newark and meet Fawn. Cassie is where you were at the beginning of the game - in the abandoned town. If you gave her fish, she'll be in her house, if not, she'll be in the warehouse where you first met her.
There is a guard at the passage from the Newark slums to Newark itself. He demands a fee for passage (5 credits). If you do not pay him, but push him aside by force, or deceive him, he will shout threats after you. You need to take a walk somewhere, and then return and go through the same passage. Don's men will detain you and take you to his house. You will need to go up to the second floor and talk to Don. He will offer you to pay a fine (200 credits). You need to refuse to pay, and then offer to do hard work for Don instead of a fine.
Don will show you a photo of Cassie and offer to find her for a reward. You need to go to Cassie and talk to her (a new question will appear). After that, if you sleep in Cassie's house, she will disappear by morning - she will be kidnapped by Don's men who were following you. Returning to Don, you will see her. It is better to refuse the reward (500 credits). After that, you will need to save Cassie. To do this, first go to Fawn's apartment and talk to her.