Theres no reason to wait for T to start gaining muscle and losing fat. I had a muscular build long before starting T. Get a good clean healthy diet, which includes the right balance of fats/carbs/protein and eating 5x per day with the correct caloric intake for you. Get a good workout routine involving cardio and weight lifting. Boobs are fat (though yes its true there is breast tissue but that based on genetics and won't increase or decrease based on exercise or diet, just the fat in them will). The more fat you lose and the more muscle you gain, will lead to decreased size in boobs, hips, butt, etc. It takes a while to do this so don't get frustrated. Like I tell people a workout forum I'm on... You didn't get your existing body overnight, don't expect drastic changes over night from working out. If it was easy, everyone would do it. It takes hard work and commitment but you definitely can start now and it will help you down the road. There are no negative effects you will gain from doing any of this pre-T.
Think of it like this... women have a higher body fat percentage than men. The most undesirable things for FTMs regarding body composition are those areas that the body fat settles regardless if you're overweight or not.