Hi @Nick
If you use the search function that the forum provides you can find answers to most beginner questions. To start you off you can read carefully this thread of posts.
The answer to your question, as you will see, is not so easily or quickly explained and it would help your progress greatly if you would take the time and make the effort to learn some “Greek”. 
The blues harp is a deceptively simple instrument that is a devil to master.
Regards,
– Slim 