You need to learn all you can about using Flash, both from the visual aspects thru the coding aspects--but all that will only get you to be able to design Flash sites... to design beautiful Flash sites requires additional talents in graphic/artistic design and innovative thinking--very often these are skills one cannot ;earn as much as they have a gift for it that they have used throughout their lives in different forms.
Your best bet will be to get formal training. There is alot you need to learn and you should expect it to take a long time to become competent in using Flash.