It’s a very good idea to take up Yoga at an early age. In fact, there is something new nowadays, called Yoga for kids or Yoga for children. Since you are a teenager we presume you would be mature enough to grasp Yoga poses and other finer points more easily. So let’s start by understanding the basic aims and objectives before getting down to real Yoga practices.
Yoga aims to achieve optimum balance, first of all between the different systems of the body such as the respiratory system, the circulatory system, the digestive system the nervous system and so on, which work so much at loggerheads with each other, resulting in so many unwarranted outcome. Next, Yoga aims to balance the body, mind and heart, i.e. between how you feel physically, in your thought process and emotions.

Coming Down To The Techniques, Yoga Is An Eightfold Path Consisting Of The Following:
1. Yamas (Don’ts)
2. Niyamas (Do’s)
3. Asanas (Physical exercises and static poses)
4. Pranayama (Breathing Techniques)
5. Pratyahara (Control of the senses)
6. Dharana (Concentration)
7. Dhyana (Meditation)
8. Samadhi (This is state of super-consciousness that is more the result of the practices than a practice itself)

What you should start with is the practice of Yamas (Don’ts) and Niyamas (Do’s). These are a set of morals and ethics, prescribed for better over all health. The Do’s consist of:
Ahimsa (non-violence in thought word and deed)
Satya (truth again in thought word and deed)
Asteya (Non stealing)
Brahmacharya (briefly, walking in the ways of the Lord)
Aparigriha (non-covetousness

The Don’ts are:
Saucha (cleanliness or purity of thought, word and deed)
Santosha (contentment with what ever you have)
Tapas (strive to tolerate and put up with the situation you are in)
Swadhyaya (study of Yogic texts and self-study) and
Ishwara Pranidhana (surrender to the will of God)

In addition you can start practicing a set of Asanas (yoga poses) and pranayama (simple breathing exercises). While the breathing exercises are done sitting, you do the asanas in the following positions:
Standing
Sitting
Supine
Prone
Inverted
Balancing

To learn these we suggest you enroll into a good Yoga institute in your neighborhood. Finally, we suggest you start having a Yogic diet. This may call for a little effort – like every new endeavor in life – but the results are very good. A Yogic diet is pure and simple, consists of light, easily digestible food and is predominantly vegetarian. Above all, please resist the temptation to start learning Yoga from books, CDs or DVDs. One small mistake could cost you much in the long run.