I don't think Tom Riddle picked Ginny. Lucius Malfoy did, when he put the diary into Ginny's cauldron with her other books. Malfoy picked Ginny because she was very vulnerable, with her age (it was her first year at Hogwarts and she was scared out of her mind), and with her vulnerability being the only girl in a family full of boys. Ginny was an easy target for Riddle to sink his teeth into.