Catchlights are simply the small white spots that are "caught" on the pupil/iris of the eyes that are reflections of the light source. You generally try to position the subject so that these lights hit about at the 1 o'clock part of the eye. If you are using natural light, you would position the subject until it is right, if using strobes, simply move the light until it hits just right. There are also modifiers that you can attach to strobes so that the catchlights are crescent moon, star, etc shaped. You can also use ring attachments to the strobes which produce a doughnut shaped catchlight, which I think look a bit odd. I think with those you try to center the light around the pupil.
Here's an old shot of my daughter that utilized natural light where the catchlights are from light coming in from a window.
You don't need to contribute much to the exposure to get a catchlight. If you are bouncing a flash off the ceiling then rubber band a white credit card to the back of the flash and this will cause a small catchlight (and eliminate racoon eyes). In a studio ask your subject if they can actually see the flash without moving their head. If so then you will get a catchlight. A photo can be ruined by having a flash just a few cm too high and not generating a catch in the eyes. It makes the subject look dead.