Girl at Heart by Kelly Oram

As the daughter of a successful Major League pitcher, Charlie Hastings has baseball in her blood. Unfortunately, being the only girl on her high school baseball team, Charlie has always been just one of the guys.
When her best friend, and secret love of her life, asks another girl to the prom, Charlie is devastated. She’s tired of being overlooked by boys because she’s not like other girls. Suffering a massive identity crisis, she decides to hang up her cleats and finally learn how to be a girl.
But with only two weeks until the state championships, the Roosevelt High Ravens can’t afford to lose their star catcher. Team captain Jace King makes her a deal: Don’t quit the team, and he’ll help her become the girl she’s so desperate to be. After all, he’s got four sisters, one of whom happens to be a cheerleader. He knows a thing or two about girls. (And if he can win her heart in the process, all the better.)