You can do it in two ways. You can define a specific set of possible player choices via the "Link To" box on the right side of the Topic Info window; I think that's closest to the old Oblivion "Choices" box. Or, you can control flow by splitting the responses into branches and setting the branches to be top-level, blocking, and so forth. Using branches works well if your dialogue follows a strict outline form, like a menu, with several topics each with their own nested subtopics, all of which the player can traverse. The "Link To" system is better for more flowing conversations where the player makes a sequence of choices. Of course, you can also combine the two methods.
I don't mean to hijack this thread but save making a new topic any chance you can answer my question DreamKing?

I'm not sure if I'm making dialogue wrong, or whether it's the dialogue/quest bug that's cropped up, but I'm having an issue where topics not discussed/clicked won't make the top topic stay white. It's hard explain, but I'll try.
Created in dialogue view. I have a branch. In that branch a topic where the player selects "I have some pelts to sell". Now I have also created a topic for every pelt type with conditions for GetItemCount on each pelt, linked to from the I Have Pelts to Sell topic.
My issue is say the player selects that, has enough wolf pelts, and selects Sell Wolf Pelts. Then they later comes back and has enough Bear Pelts to sell, "I Have Pelts to Sell" won't be white but greyed out, indicating that no new info can be read from that topic. Even though the topic Sell Bear Pelts has never been seen/clicked yet.
Am I making my topics wrong in some way?