Cave Johnson's pre-recorded message gives us a little more background on Aperture's history, which involved kidnapping astronauts, then homeless people.
Up ahead, there is a large area with a door into a lobby/office. The ground floor office doesn't go anywhere. Back out in the salt mine cave, look up on the big wall to see a metal walkway with a white wall behind it. Make a portal up there, then go through a portal of the other color to end up on that walkway.
You can go into the office area in here. This area leads you to some easter eggs. There is a sliding door behind some furniture that you can reach by jumping over the box in the way. You can interact with the box next to each vitrified door to hear a message from Cave Johnson. Out back you find a drydock where a life preserver says "S.S. Borealis." This is all just easter egg stuff though, so go back to the salt mine cave.
Make a portal on the angled wall across from the control room ledge, but make sure that you put the portal to the right of the pipe, and put it as high up on the wall as you can. Then make a portal of the other color on the floor below. Drop in. You can then walk into the control room (if you bumped into the glass or otherwise didn't make it, be sure to put the portal on the angled wall to the right of the big pipe, as high up as you can put it.)
Go into the control room and you hear a familiar voice. It's your old arch-nemesis, GLaDOS. Go over to make the bird leave GLaDOS alone (in her current form she is sometimes known as PotatOS.) She'll attempt to convince you to pick her up so you two can stop Wheatley from destroying everything. So press the button to pick her up. You'll skewer her on one of the pokey things on your portal gun, and GLaDOS will stay with you from now on. But the magnesium in your portal gun will overcharge her and she'll temporarily be out of commission.
Picking up GLaDOS also has the effect of activating the elevator. Go out there and put a portal up on the high wall, then go through a portal to get up there. Put a portal on the floor below you, then turn around and put a portal of the other color as high up on the wall across from the elevator as you can put it. Drop through the floor portal to be thrown over to the elevator.