Eleven was big. Winnie got a new best friend, and a new worst friend. But twelve is going to be huge. Last year everyone else changed, but now it's Winnie's turn to ''develop.'' Ack! Twelve is going to be a big year for Winnie, she just knows it. After all, she's one step closer to being a teenager, but there's just so much to deal with: pierced ears, sleepaway camp, junior high. . . .Can Winnie handle the pressure? And most important, can she handle bra shopping with Mom--in public?