The quickest way is a Fastrack course... there are plenty of ASA courses that get you up to Yachtmaster Coastal or Offshore up and down both sides of the US for a decent price... you can even find them on big lakes offering up to Coastal level certs... Unfortunately though from what I can tell ASA isn't nearly as recognized as RYA/MCA... of which there are no Fastrack Yachtmaster courses in the contintental US (that i can find)... though you can find a few in the Caribean.
So in summary... if you just want to learn for yourself I'd go with one of the man ASA schools available all over the US... but if you want to work outside the US I would say go RYA/MCA as it is internationally recognized unlike ASA....
but then I'm no expert.
“The world turns aside to let any man pass who knows where he is going.” (Epictetus 55 - 135 AD)
"To see new things, and live day to day, is better than wine or poppy, and fitter for a man." (Theseus)