Up ahead you find four turrets behind a light bridge. Don't go around the corner because there's a turret right there.
Create a portal on the wall where the light bridge is going, then create a portal on the wall to the left of the light bridge generator. This will create a bridge in front of the turret around the corner, blocking it from seeing you.
Now slip through the portal that the light bridge is going through. Quickly step behind the turret to avoid letting it see you.
In the big area beyond, there are four turrets guarding a companion cube. Create a portal on the column on the left so the light bridge goes through it and shields you from the turrets. Go to the far end of the room.
There are some turrets behind glass, and there are two springboards on each side. Create a portal above the wall of glass that has turrets behind it. There is a platform up above. Your objective is to land on the platform up above. To do that, create a portal on the wall above the glass that has turrets behind it such that the light bridge comes through and creates a barrier. If you position it correctly, you can use a springboard and bump into the barrier, landing on the platform up above.
Walk to the end of the walkway and you'll see the four turrets below. You have to fall behind the turrets and then knock them over. Be careful not to get shot. Try grabbing the cube and hiding behind it.
Take the cube to the big button, but beware: when you activate the button, the glass between you and the last two turrets will lower, so they'll see you and shoot at you. Create a portal behind the turrets and another somewhere that you can reach. Go through and use the exit.