Your path will be blocked up ahead, but an orange portal will appear on the wall on the other side, so look down and create a blue portal on the floor below you to reach the main part of the test chamber.
The exit has a red button keeping it closed. You can create steps up to the exit area if you step on the red button near the entrance, but the stairs disappear as soon as you step off.
There are also a couple of cube tubes that you can drop cubes out of, although you can only reach the lower one right now.
Create a blue portal on the floor below the cube tube that you can reach the button for. The cube will fall through and land near the red button near the entrance. Place the cube on that red button to make stairs appear that lead up to the exit door.
There are two small buttons up here that you couldn't reach before. The one on the right pushes the floor up for a short amount of time. The one on the left makes a cube drop from the tube onto an angled wall.
Create a blue portal on the angled wall, then press the button to drop a cube, then quickly press the button to make the floor go up. The raised floor will stop the cube from falling into the poisonous water.
Drop down to the lower area and pick up the second cube and take it up the stairs. Drop it onto the red button next to the exit, then go through and go down to the elevator.