Do you mean how do I create a mirror material?


If so create a phong or blinn, change the colour to black, up the reflectivity to 1 and assign it to the object that you want, make sure that you have an environment for the mirror to reflect either by connecting an image to the reflected colour or by putting the object inside an environment and enabling raytracing, otherwise you wont get any reflections (since there wont be anything to reflect).