Watch the scene at Shinra HQ, and when you wake up you’ll find yourself in the sewers. Before talking to Tifa and Aeris, climb up the stairs to the south and pick up the Potion. Now talk to Tifa and Aeris. After a while a boss will attack…
Fire works well on this guy, as does Bolt but to a lesser extent so use those as well as the usual Limit Breaks and attacks, this battle isn’t too tough. He has a Sewer Tsunami attack that does hit everyone but also damages him too, sometimes it will be reverse, as in it hits the bad dude first and then your allies, in this case it will hurt him more than it will hurt you. If its not in reverse, he will jump up and down a bit before casting it. You only get a measly Phoenix Down when you win.
Climb up the ladder and head south and right along the walkway. Climb down the ladder here and go across and up the stairs on the other side. Pick up the yellow Steal materia and go down the hatch, by picking the top option. Go south and down the stairs. You might want to push Select to see where to go up on the other side as it is kind of hard to see. Go up the ladder to the right to get up into the Train Graveyard.