Thanks for all of your detailed explanation.Now I don't have to worry about what I used to do.I'll try my best to improve my English.

I agree. im american and I can understand almost all British and Austrailian accents

To native speakers of English it seems crazy that learners think that US and British English are so different. If you learn English you will be able to communicate with every native speaker of English, no matter what country they come from. It is not necessary to think you have to learn American English to communicate with Americans, and British English to communicate with Brits, Australians, Irish people etc, it's all just English. The written form of the language is to all intents and purposes the same. There are some minor spelling differences which do not impede communication in the slightest. There are also some differences in terminology, but these differences also do not impede communication by and large. As with Theresa's simple example, yes in the UK we call it a car 'boot' instead of a 'trunk' but we know full well what 'trunk' means in that context in American English. With spoken English, the pronunciation is different but unless it is a strong regional accent communication between speakers of English from different countries is not a problem. As a learner you just to have familiarise yourself with the spoken English from different countries, so you don't have a situation where you can only understand certain accents.

Most people can understand and read both American and British English no matter which one you first learn. Some words for things are different, but more words are the same. US English we say "trunk" for the place in the car where you store things under the hood. British English calls it the "boot". Words like gray and grey are spelled a little different. US football is vastly different from soccer. In most other countries football = soccer. No matter which one you start out with you will be able to understand them both when listening and reading to standard English - British or US or Australian. Now, there are US English accents and British accents. Those can sometimes be difficult to understand. I live in the US and sometimes I cannot understand what people with strong southern accents are saying. I watch a lot of British TV. I sometimes have to put the subtitles on depending what UK accent the person is speaking.