Simple harmonica isn't too difficult once you realize that harmonicas come in different keys and the pattern of in/out notes are arranged in blues progression chords and transition notes. Most single harmonicas that you just buy off the counter at the music store are in the key of "C". I usually keep a "G" harmonica around myself. Just chord it in key and match it with a guitar rhythm and you sound like Bob Dylan. As a praise team, we do a verion of "Shout to the North" with guitar and harmonica that makes it sound just like a Dylan song.
Simple chording - (low notes on left) blow the first 3 holes on the left for the the Tonic or I chord. The same goes for the next 3 holes, and the last 4. Anywhere you blow, you get some variation of the tonic chord. On a "C" harminca, that would be a "C" chord.
When you draw (opposite of blow) you need to be more careful. The first 4 holes are your Dominant or V chord. So are holes #7-8 (think power chord). Holes 5-6 and 9-10 are your Sub Dominant or IV chord. There are more subtle combinations of holes... but those will get you started.
Once you get the hang of it, you'll want to become more versatile... think of getting a set like this:
Amazon.com: Hohner Piedmont Blues Harmonica Set: Musical Instruments