Why was The Glade created?  How did the Gladers end up there and how were they selected? 

The Glade was created to find a cure to the Flare; they would choose certain boys who were immune ( most of them anyway) and had over average intellegence.  They would then study their brain patterns to find a way to transfer their immunity to others.  The Creators tested them for years before eventually putting them into the Maze and wiping their memories away.  Then, they would wake up in The Box, to be transported into the Glade...

The Glade is a relatively small area that serves as a living place for the Gladers. The Glade has four large Walls that encase it and separate it from the Maze. Each wall has an opening in its center, called the Door, that closes every night and re-opens every morning. Outside the Glade lies the Maze with many twisting and turning paths that contain horrifying slug-like, spike-covered creatures called Grievers. In the book, The Glade and The Maze are actually all underground beneath the Earth, explaining how The Glade was never affected by the Sun Flares.