It really sounds like you want forum members to design this for you. Do some research yourself and I'm sure you will learn, trying to do voice recognition on a first project is just plain silly. If you can make your robot after doing the research needed, a simple electret mic and a whistle would do the trick, one whistle for forward 2 whistles for back. So thats either one pulse or 2 pulses in the circuit YOU design. I can assure you no-one will come in and design this for you, but if you do show some input to learn and SHOW what you've done members will help.
It is up to you and no-one else for your project so go do some research then come back here with what you have done and maybe if your nice members will help.
Regards Bryan