Let's talk about what we are going to do before rushing to it. Yes, we are making a robot but let us learn something first so that we can teach others too.
Okay, to make a robot talk we can go through two methods (a) Speech Synthesis (b) Prerecorded audio. Among them option a doesn't perform understandably well with Arduino. So we are going for method b.
So, we make some Audio file (.wav) and save them into a SD card. The robot will have some joints to move with the help of servo motors. And some sensors to interact with the outside world. If the sensors detect something it/she will play the audio files. Like if I point at eyes she reacts and moves her head. Let's go!