A quick summary: For beginners choose a C harmonica: Nearly all the methods (paper or web-based) use this key. You will be able to play a good selection of trad, fold or blues (especially by playing cross harp)
Then for advanced players, you can choose a harmonica in different key, in particular D F G A E which are popular choices
If you play straight in 1st postion the key of the harmonica matches the one of the tune.
If you play cross-harp (2nd position) choose your key as follows:
|harmonica||C (Do)||D (Ré)||E (Mi)||F (Fa)||G (Sol)||A (La)||B (Si)|
|Morceau||G (Sol)||A (La)||B (Si)||C (Do)||D (Ré)||E (Mi)||F (Fa)|