Using a script would be the easiest way to do this.
You can either use Self, or AIML.
There is an example here in our script library,
It is used by the Turing bot,
For Self syntax see,
Basically you want to record a variable on the conversation to track the current state.
You could also use topics.
For AIML see,
In AIML I think you may be able to use topics for this.