Then you'll need to make the background for the fish transparent..... is it?
If yes, put the fish layer in-between the computer layer with the hole for the screen, and a duplicated computer layer with the mask mode set to Subtract.
And if that simple process has you stumped, you really, REALLY, REALLY need to go here:
Getting started with After Effects (CS4, CS5, CS5.5, CS6, & CC)