There are two main types of harmonicas: the diatonic and the chromatic. The diatonic harmonica is the harmonica with ten holes. The most well known of these is the Blues Harp. (Harp is another name for harmonica). The chromatic harmonica has 12, 14, or 16 holes. The button on the side changes all the notes from major to half steps while playing, allowing you to play all the keys on this one harmonica. This is a much more difficult harp to play and is only recommended for very experienced players. For this crash course we will be focusing on the diatonic blues harmonica.
A diatonic harmonica is a single key. As a result, there are 12 different diatonic harmonicas you could acquire. I would recommend getting a C harmonica to begin with as most tutorials on the internet are played using C.
I purchased my beginner harmonica off of sweetwater.com. It was a Hohner Blues Band Harmonica in C and only cost $6. It works great for beginners and is not very expensive, so when a reed goes out it is no big deal.