The Wizard of Oz (1939)
You should definitely know this one. In this magical tale, Garland is Dorothy Gale — a young girl who finds herself (and her dog, Toto) in the fantasy world of Oz after she's knocked unconscious by debris during a tornado storm. She then embarks on a journey to find her way back home, meeting new friends such as the Tin Man (Jack Haley), Scarecrow (Ray Bolger), and the Cowardly Lion (Bert Lahr).
In 1940, the "Somewhere Over the Rainbow" singer received the honorary academy juvenile award at the Academy Awards for her part in the movie.