Why do you want to learn both? Pick one.

yes , my teachers and professors at school and university , were both British and American and now we speak in both accents there are some differences in some words and the word stress take this link to know how to pronounce the words both in British and American accents : http://www.thefreedictionary.com/today ( you will see two flags , press it to hear the accent you want )

It is not easy. But, in my opinion, it isn't necessary either. It is important that your pronunciation is easily understandable to all English speakers (native and non-native). You should be able to speak English clearly, but if you do it with a Brazillian accent - that's no problem. It will probably sound so much nicer too :-)

i see you speak portugese, and as don't speak any I can't really compare them but apparently, learning the pronunciation of many english, or american words for that matter, is difficult as letters grouped together can make many different sounds. take "ou" for instance - though - "th-oh" house - "how-se" you - "y-oo" brought - "br-au-wt" soup - "s-oo-p" but once you hav discovered the phtonetics of words like these i suppose it isn't too bad. hope this made it simpler rather than more complicated