Roll a ball in one direction. Changing direction with rotation.

Hi everyone and thanks for reading this and try to help me.

I am new in Unity and to programing, i want that a ball roll in Z direction, and change that axis Z when I rotate the ball in Axis Y.

I know that my english isnt good so i upload an image to see well that i want.

I try to program with the Unity Tutorial Project Roll a Ball but i cant get it when i try to programing myself. I am programing with JavaScript, but if you show me in C# I can translate after.

Thank you very much.