Sector 1 is one of the eight sectors of Midgar and one of the six sectors that you are able to visit. Check out the full Midgar location page for more information on the different sectors.
Sector 1 is where the story of the game begins with Barret and Cloud jumping off the train at the train station and planting the bomb at Reactor No. 1. Check out the Train Station and Sector 1 walkthrough page for additional details.
You can pick up two 
Potion in this area.
The map below shows the location for Midgar on the World Map:
