which movie in the series did she first appear in?
"Harry Potter"
Evanna Lynch
 "Harry Potter" 
how did the girl who plays luna lovegood get her part She got the role by reading about the casting call on one of the many Harry Potter fan sites and going to the open audition.
wiki:21755548
Evanna Lynch [SEP] "Harry Potter" series (2006–2011)
Lynch credits her being cast in the "Harry Potter" films to the obsession she had with the "Harry Potter" book series. At age 11, during the release of the fifth book "Order of the Phoenix" in June 2003, she was hospitalised and her family consulted with the book's publisher and the hospital. Lynch then was allowed to leave for an hour and collect a signed copy of the book. While some have stated that her prior correspondence with J. K. Rowling was a major contributing factor behind the casting decision, this theory has been debunked by both Lynch and Rowling, confirming that Rowling was unaware of Lynch being cast in the role of Luna Lovegood until the producers mentioned Lynch's name.