Im having a little difficulty matching my harmonicas with the songs; and not only does it have to do with my playin ability; which is smlim to none, but i also jsut dont see how you pick one. I think i rember hearing someone say that the harmonica you play is usualy 2 "steps" down... ummmm wtf is he talkin about. Maybe....... if the song is in the key of D you play the G harmonica??? eh??? little help please...
I'm pretty sure Harmonicas are in concert key. D = D, C = C, F# = #, etc.

Instruments like a trumpet or clarinet are in Bb, which then you would go one whole step down to find the true concert key. G = F, C = Bb, D = C.
it depends on the position you want to play. first position: same key as song, used for melodic parts. no bends. second position: either 3rd or 5th above song key, used for blues. cross harp (or third): played 3rd or 5th above song key, works best of minor blues