Use the Force on the door to get to the next corridor. You'll see a helmet dispenser here - it'll give your characters stormtrooper helmets so they are able to access stormtrooper-only areas. Each time when you're hit the helmet will fall off and you'll have to go back to the dispenser and get a new one.
Use the dispenser and go through the door. Repair the computer and watch a cut-scene. You'll end up with Han, Chewie and Luke. Go through the door up the screen. Defeat the enemies and turn left. Go forth until you see Obi-Wan in the back, fighting off a large group of stormies. Help him with your blasters, and he'll create a bridge for you. Use the helmet dispenser here and go to the end of the corridor, then turn right.
If you lost your helmet, you'll have to go back for it. Open the door and go forth. Go to the center of the room and use the turnstile to place the bridge so you can go further. Use the grapple hook to reach the helmet dispenser and a passage.
Defeat a large group of stormies and use the panel by the door - you'll need a helmet once again. Go to the next room and use the left elevator. Get rid of the enemies and rebuild the two panels using the bricks in the center of the room. Use them both and go along the prison cells. The one you want is near the grid to the right. Use the levers on both sides of the cell to get in there.