There's nothing to talk about. You can talk about anything. It's better to be a topic of interest to both of us. You can also talk about what kind of furniture and appliances you will buy when you get married in the future. You can also talk about food, programs, fashion and travel.
If you must talk about it, you may only know the details of the house, the car, the bride price, when to get married and the wedding. Planning something.
Farther away, it is possible to talk about children and parents.