See the Reference.
Import text file: loadStrings()
Display text: text()
Dialog tree: you need some logic here, there are several ways to do this, perhaps you can look at some open source implementation of a textual adventure game.

An option might be to make a class that has the blocks of text as well as a pointer id to other classes that are allowed to follow it. For example, the first instance of the class might have the text "Hello, what color is your hat?" and points to an instance of the class with the block of text that responds "My hat is red". If you reach a instance of the class that has no pointers then the conversation is done.