Work on your self-esteem. This is the most important step in being ambitious. If you do not believe in yourself, no one else will. You must believe that you can obtain anything you put your mind and efforts toward.
Decide what your goals are. You should write these dreams down in a notebook and list them in categories such as "Long Term Goals" and "Short Term Goals." I can something such as getting the promotion, or finding the right person to marry.
Keep the goals reasonable and achievable. The goals should not be so hard and bizarre that they will never be reached. By not reaching goals, you may become disappointed and side-tracked and eventually give up on all your goals.
Figure out how to improve your life and circumstances. If one of your goals is to become a doctor, take more medical oriented classes. If you want to be a professional athlete, participate in summer camps and leagues. If you would like to be a singer, sign up for voice lessons.
Work towards each of your goals and as you obtain them, highlight them in your notebook. Every time you meet a goal, add another in its place.