There are many ways to achieve catchlight. Googling will definitely give you most if not all the ways. But here is some short pointers.
Catchlight is the reflection of some light source on the eye.
Therefore the light source must be placed in the line of sight of the subject (that is the subject must be able to "see" it even when looking towards the camera).
The light source need not be "light generating" (that is a light bulb, or flash, or window), it can be a white (or close to it) reflective surface of appropriate size as well.