I have just bought a mini harmonica, and I am trying to learn how to play. I learned one or two easy chords by searching online. However, it is still challenging for me to play other chords.
My question are: "What is the best way to learn how to play harmonica for the first time? Should I learn the theory first? And if so, what is a good books or website, or how would I find one? Or should I just learn the chords and go with it? What are the benefits and drawbacks to each approach?".
Harmonicas can't play any chords other than the tonic and dominant chord of their key. A C harmonica can play a C major chord and a G7 chord.
When blowing into the harmonica, the ten holes produce the notes C-E-G-C-E-G-C-E-G-C. The notes C, E and G are the notes of the C major triad.
When drawing air through the harmonica, the ten holes produce the notes D-G-B-D-F-A-B-D-F-A. If you take the first 5 holes, G, B and D are the notes of the G major triad and F is the seventh. In the last 5 holes there are two As and the notes B, D, and F. B, D and F from the B diminished triad. While this is technically a third chord that the harmonica can play, in a C major context it will in most cases sound like a G7 missing a G.
The A note on the harp does not fit in any major or minor triad that it can form with surrounding wholes. So it will mostly be used as an 9th in the G7 chord.
One more thing you may be able to do on a harmonica is to bend an existing chord to get new chord, C major can be bent to B major on the upper 3 holes. And at the bottom, the G maj chord (G7 excluding the F) can be bent down to Gb maj with varying degrees of success because the bending needs to be even across all holes. Unfortunately these cords are not part of the C major key.
So for all practical reasons, the harmonica can only play two chords. It would have been amazing if you could play a sub-dominant chord (F maj when in the key of C maj), it would have really added to the harmonica's blues capabilities.
If by mini harmonica, you are referring to the 4 or 5-hole harmonicas, they can be seen as a subset of the ten-hole harmonica, and can also only play the tonic chord and dominant chord (the dominant chord may be missing notes in some mini harmonicas but it would still function - i.e, be heard - as a dominant).
The harmonica is not a chord instrument, it is used for playing melodies that can often be harmonized by blowing into adjacent holes. It can only play the I and V chord (tonic, and dominant chords) which can be useful in a limited set of circumstances.
How to learn harmonica as a beginner (in my opinion)
Try learning to play the scale of your harmonica, and practice playing clean notes (blowing/drawing on one hole at a time as much as possible).
For the basic technique, and basic theory, YouTube can help a lot. There are plenty of videos for beginners such as this one.
After learning your basic technique, and the scale of the instrument, try learning a tune or two that you like.
There are plenty of resources online that explain the basic theory of keys, scales and chords. You only need to learn the basic theory once you get the technical basics down (how to hold harmonica and play notes on it).
The next step in my opinion would be ear training, to allow you to recognize the notes that you hear, and be able to improvise and play by ear. At the same time start working on slightly more advanced techniques like bending and vibrato (you can learn these on YouTube as well).
