I need to be supple to practise
No: You practise them in order to become supple and healthy.
the classes suitable for beginners?
Yes: You will be encouraged to practise at the optimum level for your experience and ability.
long before I will feel the benefits of
You will experience relaxation during and after the first class you attend. Students often report experiencing a better nights sleep straight away. Benefits to general health and fitness are experienced in 3 weeks to 3 months depending on how much you practise at home. There seems to be no limit to the benefits of practising Yoga and there are always new things to learn.
Yoga a religion?
No: not a religion, but the word Yoga is etymologically very similar to religion. Yoga comes from the Sanskrit root yug (to join).
Religion comes from the Latin re(again) and legere (to join). So both words carry the same basic meaning.
Yoga, Religion and Philosophy share the common ground of a search for Truth and pursuit of personal and social wellbeing and integrity. You will find that practising Yoga deepens your experience and understanding of your religion or philosophy.
Christians practise Yoga?
Yes: Yoga deepens your experience and understanding of your religion or philosophy. It never conflicts with beliefs that are based on experience; it also brings you close to the experiences of the authors of sacred texts and the great classics of mysticism. Some Christian Yogis believe that the Bible contains references to Yoga. E.g. Take my yoke upon you. My yoke is easy and my burden is light. Burden can mean message and light can mean enlightenment.
you have to be vegetarian?
No, but practising Yoga increases you links with all life and you may decide not to eat meat or fish. The classic Yoga diet in India is lacto-vegetarian and rather difficult to follow in the West. However, moderation and sensible healthy eating are recommended.
you have to believe in reincarnation?
You need only believe that practising Yoga may benefit you in some way.
A belief in reincarnation can help you to make sense of life, but the major world religions and Patanjalis Yoga Sutra all teach the importance of acting correctly in this present life which is the only one most of us know.
want to be a Yoga teacher; can you help me?
Probably not; it depends on why you want to teach Yoga. Yoga is all about getting beyond your ego; so you have to give up wanting to be a Yogi or Yoga teacher before you are fit to teach Yoga.
The tradition is to be a student until your teacher tells you to teach or to train to teach.