Lifting is no problem at 14 years old – providing it’s done correctly. I suggest using lightweight or body weight exercises. You don’t have to push it with the weights, particularly at 14. Train the movements. That’s what’s key.
I started lifting at 13 years old, when I advanced from the “Little League mound” (45 feet) to the regulation mound (60.5 feet). My dad and I would go to the local fitness center before school two or three times a week.
At that age, I did 3-pound shoulder conditioning circuts (raises front, diagonal, and side), squats with body weight and light dumbbells, walking lunges and step-ups using body weight, push-ups, sit-ups, and, of course, running/sprints at various distances. I worked out with a trainer every few months as a sort of “check up” on my form. My dad wanted to be sure I was doing the movements correctly.
I also attended a couple of workout classes geared to younger people at the local YMCA to learn more about proper technique.
Diet isn’t so important at your age. I wouldn’t focus on it. The key is eating fresh foods with color. The more color, the more nutrients – and the better for you. Eat all other foods in moderation. (That goes for high-protein foods such as chicken and red meats, too, by the way.) Remember to drink plenty of water. Thats the one diet mistake most young kids make: They don’t drink enough “no calorie” or “low calorie” fluids –– such as water, diluted Gatorates and PowerAdes, 100% fruit juices, and skim or 1% milk.