Often we use a lead-in. So for example "Last week we were talking about gold and how people value it in society. This week, we want to talk about other things that people value in a different way, such as family and friends."
You can use phrases like "Our topic for the day is...", "Our subject for today is...", "Today's talking point is...", "Today we are talking about..." "So, what are we discussing today?" etc.