Go straight from the elevator and go through the door beyond the room number sign. Keep going forward to hear from GlaDOS and Wheatley. Eventually GlaDOS will open the next door.
Go through into the main testing area. Notice that there is a cube chute, but if you press the button, the cube falls into the toxic pool below.
Create a portal on the wall next to the light bridge. Then create a portal on the wall across from the door that you came through. Go through the portal to end up on the light bridge.
Once you're on the light bridge, create a portal on the wall across from where the light bridge is coming from. Then walk away from that portal and turn around. Create a portal of the other color on the wall under the cube chute. Position that portal so the light bridge goes past the cube chute button.
Now walk along the light bridge and go through the portal. Keep going until you reach the cube chute button. Press it. If the cube falls off of the bridge, change the position of the portal so it's under the chute.
Go get the cube. Carry it through the portal and go all the way to where the light bridge is coming from. Turn around and create a portal on the short wall that you were standing on when you pressed the cube chute button. Go back through the portal near you and walk to the exit. Put the cube onto the big button.
Go through the exit and down into the elevator to move on to the next level.