Hi
The best book for beginners is , in my view, 'Philosopy-The basics '
by Nigel Warburton. All his books are clearly written and I think any
intelligent 16-year old could understand them.
There are also a couple of books by other authors specifically for the
AS exam.
Philosophy is about examing the reasons why we believe a certain thing.
It may not give you answers but I believe it makes you a better person
and more tolerant.
Hope that has helped.
Mike