Which English do you want to speak? There isn't one, but several Englishes that people all over the world use to communicate. Of all the different styles of English spoken in the world, American English certainly dominates, being the language of Hollywood, computing and aviation. Some would even go as far as calling their language "American", and why not? It has several important differences with my English, British. But for a foreign learner to say that they speak English and American is exaggerating somewhat. The biggest differences between British and American English are vocabulary items, just as there are regional differences in any language.

Learning American Words

If you have studied English in a traditional class, the chances are it was based on British English. Even American teachers have little choice but to use the resources available to them, and Oxford and Cambridge University Press dominate the world of ELT. If you are studying a course like English File or Headway and your teacher is American, you have a good advantage, as likely he or she will use his American vocabulary in conversation.

The best way to familiarise yourself with American vocabulary is simply to watch American TV and movies (films in British English!). Generally, you will be able to see from context what a word means, and link it to the British word you already know.

Pronunciation

There’s a famous song that highlights some pronunciation differences, the word “tomato” for example. Many of the differences in vowel sounds can be attributed to the change in stress patterns. In the US, “Iraq” has its stress on the first syllable, pronounced like “eye”. In Britain, on the other hand, the “I” is not stressed so it’s pronounced like the “I” in “it”.

Grammar

As for grammar, the good news is that Americans use less and less the dreaded present perfect - so you have one less thing to worry about. Words like "just" "ever" and "already" can be used with the past simple, whereas in English it's the present perfect.

British: have you already seen this film?
American: did you already see this film?

The British generally have no problem with American English as they are used to watching American films (or movies if you prefer). An American, on the other hand may have a few difficulties understanding a Briton, especially if he uses slang words.

Jonathan Lewis
http://anglais-facile.com/index2.html