Triss Merigold is one of the romance options available to Geralt in The Witcher 3, and you can begin her questline upon your arrival in Novigrad.
To begin her questline, start by completing the various main story missions in Novigrad until ‘Count Reuven’s Treasure’. After finishing this one, Triss will tell Geralt that she has a favor to ask of him and that he can find out more by visiting her at her house. Triss will inform you of her plan to get all the mages currently residing in Novigrad out of the city, and that her next evacuation target is Lady Vegelbud’s son. This will unlock the secondary quest ‘A Matter of Life and Death’.
This is a fun little quest that requires you to buy some fancy clothing and a fox mask so that you and Triss can gain entry into the Vegelbuds’ masquerade ball. During the course of this quest, Triss will get drunk after like, three sips of wine (go figure) and lead Geralt into a hedge maze. Do note that if you wish to romance Triss, you have to select the option to kiss her when you’re inside the maze. If not, feel free to choose the other option and the quest will continue as well.