kmitar,
first off, i am not a personal trainer or medical practitioner of any sort... nor do i have any certifications in fitness or nutrition. however, i do have over 12 years of personal experience with weightloss, dieting and physical training... and have been very successful at finding plans/routines that work very well for me, as well as other people who have solicited my advice. i have done extensive research over the years in my own quest to become "fit" and believe i can offer some sound, safe and healthy advice. if you'd like to hear, read on. if not, i understand.
i do agree with some of the other responders in that you are still young and growing. to take any drastic measures to lose weight would be premature at this point in your life. 150 is definitely not obese for someone 5'9"... and is arguably not even considered to be "overweight"... regardless of your gender. however, i would say this to you: there is nothing wrong with wanting to be a healthier person... no matter what your age, shape or size is. "healthier" being the operative word here.
my advice to someone like yourself would be to not concentrate on losing weight or defining muscles. concentrate on making changes in your lifestyle that will promote a "HEALTHIER YOU". like others have said... get involved in an organized sport... or take up jogging, walking or fitness kickboxing. inject a physical activity in to your daily/weekly routine and concentrate on making it a part of your lifestyle. once you do this, you will become more interested in fitness as a whole and natrually educate yourself on healthy ways to become/stay fit.
on the topic of dieting and meal plans... i feel VERY STRONGLY that this goes hand-in-hand with a physcial fitness routine. they feed off of each other. if you concentrate on one, the other will usually follow... and you will find yourself become more disciplined in both areas as you learn more and progress.
again, its about becoming HEALTHY... not about losing weight, building muscle, getting a six pack, etc.. especially for someone like yourself. all that stuff will happen if you dedicate yourself to the bigger picture. use the internet and books for information/advice. there's a wealth of knowledge out there. go about it in a responsible, healthy way and i guaranty you will begin to feel better about yourself.