If you want to play in 1st position, just pick a harmonica in the same key as the song.
If you want to play in 2nd position, count up 4 from the key of the song. If the song is in the key of G, and you want to determine which key harp to use for playing in 2nd position, just count up 4 from G: 1-"G", 2-"A", 3-"B", 4-"C". A "C" harmonica can be played in 2nd position to play in the key of G.
Here is an article on this topic.
Check it out