How do you play a harmonica that has 24 holes?

my boyfriend surprised me with a Weekender Hohner International (Tremolo harmonica). the thing is neither of us know much about the harmonica, and there's no where on the box or the harmonica itself that says what key it is in. plus it has 24 holes. is there a certain way i have to learn how to play it or is it the same as a harmonica with 16 holes? also how can i find out what key it is in?