You have 3-4 options for obtaining a copy of this text.
"White Angel" is a short story that was first published in the July 25th edition of The New Yorker in 1988. In order to read the full story online, you would need to purchase or use someone's subscription to access all of it. It is possible that a teacher in your school or a local librarian could get this for you. You might also check your local library to see if they have back copies of this magazine (as many do).
Cunningham's novel A Home at the End of the World, published in 1990, is the expanded version of this short story. The short story is one of the first chapters of the entire novel. Finally, "White Angel" was later selected for publication in The Best American Short Stories of 1989, which can also be found at many public libraries.