In this test chamber, the computer warns you about the poison floor. You'll notice, looking through the glass, that there's a portal gun down there, shooting orange portals. It would be awesome to be able to make both blue and orange portals, wouldn't it? But first, you have to reach the gun.
You might notice that you can't safely go through any of the orange portals that you can see the rotating gun create. You can only see it make two because it avoids the wall with the pellet containment thing. The other two portals would drop you into the poison floor. However, the gun also creates one orange portal where you can't see it. That's when you want to go through it. So, make a blue portal on a wall near you. Look through it to see where the orange portals are being created. Once the portal is going to dump you out in a place with a floor, go through.
Use the E key (or whatever you set your pick-stuff-up key to be) to press the red button. This very briefly opens a door in the wall to a different area of the chamber. You need to create a blue portal on the wall by shooting through that door while it's open. Then, when the orange portal gun shoots an orange portal onto the wall near you, go through it to get to that other chamber.
There is a high energy pellet in here. Wait for the orange portal gun to create a portal on the part of the wall where the pellet containment thing is shining its light. Then shoot a blue portal onto the wall where it hits the wall. The pellet will go through and into the containment thing. This starts the unstationary scaffold.
Now, make a blue portal where you can walk through it, and wait for the orange portal gun to create a portal on the wall where the scaffold stops. Look through the portal to ensure that you go through it when the scaffold has stopped at the wall so you can step onto it. Then, after the scaffold takes you to the center, walk up to the orange portal gun to get it. Your gun can now make both blue and orange portals. Yay! The computer unsettlingly informs you that the gun is now worth more than the combined organs and incomes of everyone in 'subject hometown here'.
Press the red button that appears. It will open the door on the wall above. While it's open, create a portal (doesn't matter which color) on the wall beyond the temporarily opened door. Then create a portal of the other color on the wall where the unstationary scaffold will go to. Step onto the scaffold and go through the portal. You can now go into the elevator to the next chamber.