[Psycho_Coder]

This is not an easy task. There are competitions and there is even an award (Loebner Prize) for the creation of programs that are able to answer like a human would.

But for the beginning it will be enough to do it like ELIZA: https://en.wikipedia.org/wiki/ELIZA
Her implementation is simple and you will realize that she has no memory of what you told here after a while of chatting.
Here is site where you can try chatting with her: http://nlp-addiction.com/eliza/

Also interesting: http://alicebot.blogspot.de/

For more bots look at this article: https://en.wikipedia.org/wiki/Chatterbot

Also NLP (Natural Language Processing) is an interesting topic for you, if you want to get deeper into it: https://en.wikipedia.org/wiki/Computational_linguistics

My friend told me about ELIZA and it sure is interesting. I did have some fun with it and got really astonish with some replies. (I was being naughty, hahahahahhah)