1. trim the ends of the light layer in the timeline, or
2. animate the intensity of the light layer using keyframes.
The manual should give you the details. Look for sections on working with layers, animating with keyframes.
If you don’t want the lit layer (not the “light” layer) to disappear when you dim the light, try adding a duplicate of the lit layer into the timeline with “accept lights” off. Animate the opacity of that layer so it begins to appear as the other one dims … if you want that effect of course.
Since you’re a noob, I’ll fill you in on a couple of things: most people here prefer that posters read the manual and search the AE online help before asking questions. The COW is not a substitute for the manual or the online help. However, we do understand that newbies don’t know all the search terms that apply to their situation. (“track matte”, for example) In such cases, the newbie will often be given the search term, then directed to search the COW posts (to your upper left on this page) or to search the AE online help or manual.
This is an extremely high-traffic forum. If it were filled with basic questions, the more difficult questions would get lost and the more experienced posters and leaders would lose interest.
In other words, try the manual, the online help, and some experimentation to get to know the program before you post.
Hope that helps,
Steve