Just one word - practice!! I have been learning for 5 months now, and I don't have the full stretch on B and F yet. I'm playing half chords whilst trying to get my fingers used to the stretch. The string muting and buzzing is down to finger positioning, so make sure your fingers are straight down on the strings and you're nail isn't touching the next string. Again the more I practice, the better it gets and the less I have to think about it. Not perfect yet by any means, but I'm getting there slowly! I'm no expert, but hope that helps! :)