"Fat girls have fewer choices in life. This is painful but true. You'll never be the center of attention; never the belle of the ball. No one gives fat girls the benefit of the doubt. Skinny, pretty girls get the best of everything. We get whatever's left."   So warns the mother of Wilma "Sunnie" Sundstrom, a bright, precious, overweight 13-year-old whose lifelong dream is to meet her idol Johnny Carson, and appear as a guest on The Tonight Show. The year is 1978, and Sunnie a straight-A student and aspiring filmmaker deals with constant teasing from her eighth-grade classmates by dreaming of Hollywood accolades and writing a screenplay, "Girl on the Lam," which she is certain will star teen sensations Kristy McNichol and Robby Benson.   Sunnie lives in suburban Milwaukee with her banker father, her stay-at-home mother, her sullen older sister, Ingrid, and her brilliant younger brother, Max. Sunnie's closest friend and confidante is Grannie Lassen who encourages Sunnie's show-biz dreams. Grannie's own tap-dancing ambitions were cut short by the untimely death of Grannie's younger sister, Wilma.   A devastating loss plunges Sunnie into despair and her family packs her off to an all-girls summer "fat-camp" in Northern Minnesota, where her days revolve around skimpy meals, calisthenics, and humiliating daily public weigh-ins on the camp's livestock scale. Love enters Sunnie's life in the form of a dashing boy from the Pentecostal Bible camp across the lake. When his late night canoe visits suddenly and mysteriously cease, Sunnie and her friend Cherise take matters into their own hands, with nearly tragic results. Rather than return home in shame, Sunnie takes a desperate leap of faith and embarks on a cross-country journey. Challenged more deeply than she ever expected, she discovers the unorthodox ways that dreams really can come true.