Hi encee,
For beginners it appears that most experts recommend that you choose a diatonic harmonica in the key of 'C' as it is apparently the easiest to understand and has the most tutorials written for it. It is also one of the cheapest versions of the instrument to buy.
The first of the following links describes the different types of harmonica and the second features a recommendation -
http://en.wikipedia.org/wiki/Harmonica
http://www.harmonicaworld.com/home.php?cat=286