After you successfully avoid passing out from thirst, you reach Test Chamber 14.
Get a Cube
After you emerge from the hallway, turn left. Go up the stairs that appear, and the stairs will disappear again when you reach the top. There is a cube on a platform. To reach it, create a portal to the left of the window on the wall that faces the cube. Then create a portal of the other color in the pit where the stairs were. Fall into the floor portal to reach the cube. If you are too far away, you can jump up as you move toward the cube (go to Settings to check how to jump if you don't remember).
Cross the Poison
Take the cube and drop down to the floor. Go back toward where the stairs were, and they will reappear. Go down the stairs and put the cube on the red button. The chamber lock will open. Go through and you will reach an area with a poison pit, and some scaffolds that move up and down. You can either make a portal on the far wall and a portal of the other color nearby to avoid having to deal with the platforms in the poison, or you can carefully jump onto the first scaffold as it comes up out of the poison, then jump to the next platform, then jump onto the second scaffold as it comes up out of the poison, and jump from there to the door. Be careful not to walk too far through the door or you will be hit by the high energy pellet.
Contain the Pellet
In the pellet room, make sure that there are no portals facing you, and then make a portal on the wall where the high energy pellet has made a dark mark. Note that the aiming reticle will show you the color of the last portal that you made. Then go back to the big red button that you put the cube on, and go toward the stairs that lead up to the pellet containment device. Make a portal of the other color (i.e. not the color that the aiming reticle is showing) on the ceiling where the containment device is shining its orange light. The pellet will go through the portal that you made earlier and enter the containment device.
Now go to the platform that was lowered by the pellet containment device and stand on it to be lifted to the elevator. Go in to reach Test Chamber 15.