Garth was born in Urbana, Illinois, to John and Carolyn Garth. The youngest of seven children (though their only child together), Garth spent much of her youth on a 25-acre horse ranch between Sadorus and Arcola, Illinois. For a time the family stayed in Tuscola, Illinois; but eventually they settled in Glendale, Arizona, when Garth was around 13 years old. She studied dance and modeled, and was soon discovered at a local talent competition by Randy James, Hollywood scout and manager. She attended Greenway High School as a freshman and transferred to Apollo High School in her sophomore year. Determined to become an actress, Garth would receive audition materials from Los Angeles that she would work on with Jean Fowler, a local acting coach. She left school during her junior year to work in Los Angeles with James and would later obtain her high school diploma in California.