Find: 3 Potions, 1 Phoenix Down, & Restore Materia

Once you have jumped off the train, search the guard on the ground twice for 2 potions, run forward and you are attack by 2 guards. Then follow after your allies. Talk to Biggs and after that Barret will come running in. Enter the reactor, heading North and West through the doorway.
The next screen in a catwalk high above the ground. When you come to a T-shaped junction, go north, into the reactor. Talk to Barret, and he joins your party. Next, talk to Biggs (on the left) to open the door. Run through, then talk to Jessie to open the next door. Go north, but before entering the elevator, search on the right for a chest containing a Phoenix Down. Now talk to Jessie and enter the elevator.
After pressing the button in the far right corner, Barret and Cloud talk a bit. Exit the elevator into a large room with stairs. Take the stairs down and right, then down and left, until you reach the ground. Enter the door directly in front of you. Walk up to the gap and jump over it, then talk to Jessie. She explains how to use ladders. Climb down, and then walk towards Jessie to get the potion in front of her. Now run down the ladder on the right, and climb down the ladder next to the platform. Run down the pipe to the next screen.
Run down the pipes over the ladder on the wall. Here you encounter your first save point. Save your game. Run down the walkway to the next area. There is a green dot, which is the Restore Materia. You won't be able to use it until later. Run up to the reactor, and watch the action. Cloud places the bomb, then the Guard Scorpion attacks them. You are given 10 minutes till the bomb detonates.