Before explaining why the evening sky is red, you must first understand why the sky is blue.
In the evening, the sun, which gives off white light (which is light of many different colours), seems red to the human eye. When the sun begins to set, the light needs to travel further to reach you. The light, due to the long distance, is mostly reflected and scattered.
Point 1: As the light has to travel through a further distance, less light reaches you. This makes the sun appear dimmer in the evening.
Point 2: The wavelength of the colours blue and green have been scattered due to the distance. Colours of longer wavelength will then reach the eye. The colours orange and red.