Ignore the platforms that keep slamming into the wall and go through the entrance door.
Up ahead, some floor pieces and walls will move. Step onto the aerial faith plate. You will see none other than Wheatley, who apparently survived being crushed by GlaDOS. GlaDOS (who seems not to know that Wheatley is up there) will tell you that you broke the faith plate. She'll ask you to use it a few more times, during which you will again see Wheatley. Finally, GlaDOS will lower the ceiling for you. After she does this, put a portal on the ceiling piece that she lowered, then put a portal of the other color onto the ceiling above the ledge that you can't reach right now. Doing this will make you fall onto that area up there.
To open the exit, you will need to shoot a laser into the laser receiver. However, the only laser in the room is shooting at a non-portal wall. So you need a laser redirection cube, and it appears that there is a cube chute on the ledge above the laser shooter. The button that activates the cube chute is in the wall opposite from the chute itself.
To reach it, make a portal on the angled wall that faces the small red button. Then go back down to the aerial faith plate and use a portal in the ceiling to be catapulted through the portal to reach the small red button. Press it.
Now create a portal on the other piece of angled wall in this area, and use the aerial faith plate again to be catapulted to the cube chute. Pick up the laser redirection cube and hop down below. Jump over the laser and put the redirection cube in its path such that it shoots at the low piece of wall nearby that has portal walls and is vertical rather than at an angle.
To get up to the ledge with the exit, you will have to use portals, which means that you can't yet use portals to make the laser shoot into the receiver. Create a portal in the angled wall that faces the exit ledge. Then go down and use the aerial faith plate once more to shoot into the ceiling portal, which will make you land on the exit ledge.
From up here, shoot a portal into the wall where the laser is hitting it, then go to the other side of that wall and shoot a portal of the opposite color there to make the laser shoot into the receiver. The exit door will open. Go to the elevator.