Once I learned that I needed an A harp to play in the key of E, the rest came easy. Like others on this thread, I learned by playing along with Sonny & Brownie records. Charles Kratz has a lot of good info in his post. Like him I prefer the plastic comb. I have a few Lee Oscars, some Big Rivers and some Special 20s, all with plastic combs. I started playing on Marine Bands in the '60s when they were the best option. We used to soak them to make the reeds easier to bend and the wooden combs would swell. We'd then use a utility knife to shave them back and from then on they'd be uncomfortable on the tongue.

A recent discovery for me, although I use it mostly for straight playing, is the low D tuned 'harp. I'd always found the regular D kind of high pitched for playing fiddle tunes with a fiddler, but the low D puts me in the same octave as the fiddle for most tunes.