Mel Bay does a very good book which you can use to teach yourself but the secret is practise, practise and more practise. It's not a bad idea to start with a 12 bar which you can play in D and save your money for a big one when you know if it's the right instrument for you. Good quality 21 bar autoharps are not cheap. The big suppliers of folk instruments in the UK will get you one if you ask them and most stock 12 bars as standard.