You mean to fade in and out? That's really easy.
Make a new layer on top of everything you want to fade in/out.
Where you want it to begin fading, create
a box over the whole stage that is the color you want it to fade into.
(For example, black is the most common.)
Then set the opacity of that box to 1%. (In the fill color panel)
Then make a keyframe on that same layer about 20-30 frames away.
To the box that is on that keyframe, change the opacity to 100%.
Then right click in-between those two keyframes and select "Create Shape Tween".
There you have it, it fades in.
Use common sense to figure out how to fade it out.