Ok,

Having decided on a shotgun, made the purchase, and determined it is time to train, lets look at a light. If you mount a light, you need to train with it also. You don't want to just turn it on and go looking for the trouble. The light lets others know your coming long before it lets you know where the others are. You will probably be leaving a darkened space and your eyes will be dark adjusted. The bad guy will have come from a place of some light, so his eyes won't be as adjusted. Using the momentary on to "flash" the light will give you sight without telegraphing your approach. I am not going into the small print on weapon/light usage here, it would be a disservice to all involved.
There are plenty of books and instructors available. Now is the time to put them to use. A good instructor will teach you about safe handling, accuracy, immediate action drills, light usage, and weapon retention.

Yeah, weapon retention...

Here in DFW we recently had a home invasion where the lady of the house took the firearm from the bad guys because they were unsure of its use. In the moment of hesitation after hearing "click" (take it off safe), she grabbed the gun and handed it to her husband who had caught up to what was happening from the other room. Husband then ventilated the bad guys with their own weapon.

Look at the stats on how many LEO's are wounded/killed with their own weapon. Then weapon retention may seem more important than that light.

It would be a crying shame to see what that nice new shotgun looks like from the business end with someone else holding it.