Escape Part 1
On this ledge, you'll have only one path—left. Use the portal there to climb up the platform without using the broken steps and enter the door. Up the steel stairs again, the door in front of you will be closed. Turn right, place a portal behind the iron mesh, and place a second one in the wall next to you. Go through the mesh.
There's another door, already open. In the next room, to get through the huge fan, place one portal behind it, a second one next to you, and enter. Once on the other side of the fan, you'll see a water barrier. It can be crossed using a portal: simply shoot one portal to the far platform and place the second one nearby.
From this platform, you need to reach the far bridge. To do this, place a portal on the wall behind the bridge, higher than the bridge itself. Time it so that when you jump out of the portal, you land directly on the bridge. Then follow the bridge to the left and down the steps. You'll find yourself on a small platform with seemingly no exit.
Look at the walls; one of them will have a red arrow pointing upward. Place a portal on that wall as high as possible. Place a second one on the floor and step into it. After exiting the first portal, either try to hit the second one again, flying forward, or simply fire the cannon under you again. After that, you'll emerge from the portal on the vertical wall again, but much faster and further this time, so you'll reach a ledge.
Move forward and climb the stairs. You'll see a hole in the pipe through which the cubes travel. Jump down. After a short ride, you'll find yourself back in one of the test rooms. Pass through the force field on the left, stand on the button, and the door in front of you will open. Fire the portal at the wall behind the door. After that, place a second portal on the wall in your room and step through it.
You'll come to an empty elevator shaft; jump down. After landing nearby, you'll find an area fenced off with a net. Place a portal on the other side of the net and cross over. There, you'll find two pistons, each sinking into the floor and then rising. Stand on the tallest of them and turn around. After it straightens up, you'll see a small platform. Place a portal there.
Then, dismount the piston, open a second portal, and teleport to that platform. Walk along it a bit further and you'll see a piston, this time horizontal. Stand on it, and after it straightens up, cross it to the next platform. Afterward, turn left and you'll encounter a grate covering a ventilation shaft. Shoot through it and place a portal behind the grate.
Then, open the second portal and cross over to the ventilation shaft. Once you're there, look up; you'll see a passage in the wall just above you. Place a portal on the wall just above the passage, place a second portal next to you, and jump through the portals into the hole in the wall.
Escape Part 2
After passing through the passage, you'll find yourself in a large room with huge valves moving up and down. Stand on the largest of them, climb up, and you'll see a hole in the wall. Place a portal there, then drop down, open the second portal, and teleport up. There's another room with valves.
Shoot one portal at the upper platform, and the second one next to you, on the ceiling above one of the pistons. Stand on the piston, and it will "push" you into your portal. You'll find yourself in a small room with no obvious exit. Search above; there should be a space for a portal on the wall just under the ceiling.
Place it there, place the second one under your feet, and "double-jump" through the portals again. You'll eventually land on another ledge. Turn left on it. Next, your path will be blocked by a series of moving pistons. To overcome them, simply place a portal behind them and a second one next to you.
Eventually, you'll reach a large room with huge pipes moving randomly above you. There will be many of them, but they'll move slowly. You need to ascend slowly but surely. Place a portal above one of the pipes, place a second one next to you, and enter. Once on the pipe, look for the next one higher up, place a portal there, place a second one next to you, and repeat until you reach the very top.
There will be a ledge enclosed by a net; you already know how to get through it. There, you'll find a gap in the floor with a glass pipe running through it. Drop down and follow the pipe. When you reach a dead end, jump onto the next glass pipe and follow it. You'll see a huge piston moving from right to left, across the pipe you're walking on.
Wait for it to move to the right, step forward, and place a portal on the right wall so that when the piston straightens out, you can step onto it from the portal. Place the second portal on any wall slightly behind you and slightly below the pipe.
Once the piston is in place, jump into the second portal. You'll emerge from the first portal and land on the piston. Follow it further. When you see an arrow pointing downwards, jump down.
Soon you'll reach a room with three double doors. Stand by the wall without one. The left door will open first. As soon as it opens slightly, shoot the first portal there, the second one next to you, and enter this room. Grab the turret there and throw it aside. Do the same as the other doors open. In one of the small rooms, there's a hole in the ceiling.
Shoot a portal there to place it on the wall, shoot the second portal next to you, and teleport. In the room where you find yourself, there's a glass pipe. Place a portal above it, the second one next to you, and then cross over to the glass pipe.
Jump from it to the platform. Look for a small passage on it that can only be accessed by crouching. Climb through it. There's a turret below. As soon as you jump down, grab it and throw it on the floor. A hatch will open, and you'll exit.
After this, look around and you'll see a slanted plate. Place a portal on it, then a second portal on the floor. Then, double jump. You'll enter the laboratory. Move forward, and soon you'll reach a room with a turret behind glass. Wait for it to lock onto you, then step aside. The turret will fire and pierce the glass with a missile.
Enter the room with the turret and go to the glass opposite. Repeat the targeting trick, and once the glass is pierced, move on. Soon you'll hit glass again, so place a portal opposite it. Go back and find a good spot, place a portal, and stand in front of it and wait for the turret to lock onto you. Then, step aside.
The turret will fire a missile at you, which will hit the portal and fly out of the first portal, after which it will pierce the glass. Continue further and you'll soon see a ventilation shaft, but you won't be able to climb into it because it's too high. Place the portal again, aiming the missile fired from it to hit the glass pipe nearby.
Return to the turret and repeat the rocket trick, then return. A crate will fall out of the broken pipe; place it under the ventilation hatch and crawl through it. After crawling through the hatch, you'll encounter a fan in the floor. Place a portal, place a second one next to you, and you'll find yourself on the other side.
Escape Part 3
After this, move forward using the portal, overcoming simple obstacles. When you reach a dead end, look for a small grate on the ceiling, shoot it with your cannon, and place a second one next to you. Soon you'll find yourself in a huge room with four pillars and multiple hatches in the walls. After some time, they will all open, there will be turrets there, deal with them in the same way as shortly before in the room with three of the same hatches.
After all the turrets are defeated, look around. You need to find two slightly open tiles diagonally opposite each other. Place a portal on one of them, then place a portal on the floor and jump into it. You'll emerge from the portal on a diagonal platform and land on a ledge in the wall. Move forward, and two turrets will appear in front of you.
Place a portal behind them and a second one next to you. Teleport behind the turrets and shoot them down. Next, to jump to the ledge above, place a portal on the floor, return to the cliff, place a second portal at the very bottom, and jump into it from above.
Flying into one portal, you'll emerge from the other with enough speed to launch yourself onto the ledge. After that, continue forward and soon you'll encounter the game's boss: GLADOS, the same voice that's been bothering you all this time.
Boss
As soon as you enter the room, pay attention to the booth on the left. Place the first portal there, and the second portal next to the cauldron. After this, wait a bit; a piece of the boss will fall off. Take this piece and bring it to the cauldron. Then, enter the teleporter next to the cauldron. Once in the booth, press the button and jump back through the teleporter to quickly reach the cauldron and throw the part there.
After this, the boss will be offended and will start releasing gas into the room and firing rockets at you. Place one portal on the wall opposite the boss, the second closer to you, and try to "redirect" the rockets fired at you through the portals directly at the boss, as you did when you smashed the glass and the pipe. After each hit, another piece will fall off the boss.
Treat them the same way as the first piece. After the fourth piece is sent to the cauldron, the boss will die and explode. The resulting blast will throw you outside. But the ending of the game is very ambiguous, so see you in Portal 2!
Portal 1 guide: (Part 1) (Part 2) (Part 3)
Source: gamer.ru