What is the difference between English british and American English? What is the easiest? simple?
Most of the people learning English I've found uses to say that they prefer American English rather than British one. Well, that's not exactly true. You should listen to different accents (and have your accent easy to be understood by the people). If you just listen to the movies, that comes into American English, you will jump to the conclusión that British English is more difficult. That's because your ear is used to that accents, and it's difficult for you to recognise another ones.
So, my advice for you, is to listen to any kind of (native) accents. Choose one model of English, but get used to different accents: watch british movies, if possible from another English-speaking countries (Canada, New Zealand).
One thing is true: the variation in accents is wider in the UK than in the US (taking apart Southern accents). Pronunciation is different (the accents), but in general all of the rest is more or less the same in the different countries (Ireland, Scotland, England, United States, ...), pulling apart the regional dialects (but don't expect to understand all of the accents just because you feel very comfortable in just one place, as in London, New York, etc)
The grammar is exactly the same and, save for a few exceptions, so is the vocabulary. The main difference is in the accent. Until your English is at a level where you need to perfect your accent, you won't need to bother with the differences.
There is a mix of different pronuncations, spellings and terminology.
UK english tends to follow the traditional spellings, where US tend to be based on how they sound
UK Colour, US color
US will use z when UK uses s
Organize (US), Organise (UK)
And there are subtle differences in style. for instance, Whilst is used more in UK.
In terms of terminology there is a lot of difference. due to US TV most people understand US terms.
Like sidewalk (pavement), trunk (car boot), hood (car bonnet)
but mostly the same really.
It's not just the accents. In Britain there are a lot of idioms / expressions which are not used elsewhere.
Hi I think for you (Brazilian girl) maybe is easier learn US English, if not for other reason that you are used to the accent as well as here in Europe many people prefer UK English. As someone above said every country has their own idioms and slangs, Anyway both are great and when you will fluent you don't have many problems to deal with US or UK English.
Off-topic: the same logic works for Spanish from Spain VS Spanih from Lat-Ame., both are beautiful even Argentina that has a particular kind of Spanish.