Help me with my balls (hehe, what a title)

I have a scene with a circle in it representing a ball.
I have a border which i would not like the ball to go out of.

What i would like is the ball to set of at a speed of 10 at a random angle.
The ball will then bounce off of the border at its correct angle of incidence.

Please help.

I have attached the file i wish to do this too

Please if possible add comments to the AS to explain what you did so i can understand it.

Thanks.