I would start with a full body weight routine, lifting heavy, and a good diet. I don't know if you can change your body type, per se, but you can lose inches all over and have a more shapely figure. A lot of people like New Rules of Lifting for Women. I just started Starting Strength. It's great, and it's kicking my butt, literally. So you want to develop a basic level of strength, which established programs are perfect for, and once you have it, you can go more towards hypertrophy-building muscle, and build a bigger bottom. It's also possible that once you lose some of the fat in your thighs and waist, your bottom will make an appearance.
You probably have a lot less muscle than you think you do. Take smashley's advice and lose fat and build muscle.
I agree with Smashley too and with solid555. You will also lose some of your bust which is mostly fat with underlying muscle supporting. A good indicator of your final proportions might be your frame size. Google body frame size and take recommended measurements (usually your wrist or elbow) to determine whether you are small, medium, or large framed.
Also regarding your butt situation, make sure you do exercises targeted for your glutes. Remember working the largest muscles in your body (generally the thighs) will get fastest results of weight loss since larger muscles burn more calories. Lunges and squats good for those.
There is lots of good info on this site (activity and exercise) and online in general to get you started. Be sure and start slow and with light weights.
Good luck to you. You can do it!
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