Go forward and open the next door. In front of you is a bunch of toxic sludge. Turn left and go along the walkway. After going up some stairs to the upper level, turn around and go the other way. Soon you see a door that is broken open. Create a portal beyond this door, then a portal of the other color on the wall next to you. Go through.
In this room, go to the end toward the unbroken door and pull the lever on the left wall. Go through the door. You'll hear a recorded message from Aperture Science founder Cave Johnson and his lovely assistant Caroline. Go into the room toward the Aperture sign.
On the wall up ahead, there is a sign that says Main Lift. Look up and to the right. There is a white wall way up there. Make a portal there, then one of the opposite color nearby and go through. Walk along the metal walkway to the left. When you're standing on an angled piece of wood, turn around and look down. There is an angled white wall down below (to the right of where it says Main Lift). Put a portal on it. Then look down below the plank of wood that you are standing on. There is a white floor almost directly below you. Put a portal on that floor and then fall into it. You should fly over to the door where it says Enter. Go through the sliding door here.
You'll hear another pre-recorded message from Cave Johnson. Go through the lobby and through the sliding door at the end. Up ahead there is a walkway that is broken at the end. Jump from the broken end to the walkway on the other side.
Get on the yellow elevator and press the up button. At the top, look around for a door in the distance that says "Alpha". You need to reach this door. Look to the left of it to find a ledge with white walls and some closed doors. Notice that there is an angled wall there that is partly white. Make a portal on that angled wall. Now go down the stairs in the elevator area until you find a spot where you can look down the elevator shaft. Make a portal of the other color on the white floor below the elevator. Drop in. You should end up in front of the Alpha door. Open it and go through.
Turn left and go into the office. Pull the lever where it says Pump Station Alpha and Gel Pressure Control. The door to the right will soon open. Go through. Go forward and then turn left when you hit a dead end. Look up to see a ledge up there. Put a portal on the wall there, then a portal of the other color nearby, and go through.
Turn left and go to the big open area. Walk along the gel tube. At the end of the tube, jump to the walkway up ahead. Go through the material emancipation grill.