Differences Between American and British English
Pronunciation - differences in both vowel and consonants, as well as stress and intonation
Vocabulary - differences in nouns and verbs, especially phrasal verb usage and the names of specific tools or items
Spelling - differences are generally found in certain prefix and suffix forms
The most important rule of thumb is to try to be consistent in your usage. If you decide that you want to use American English spellings then be consistent in your spelling (i.e. The color of the orange is also its flavour - color is American spelling and flavour is British), this is of course not always easy - or possible. The following guide is meant to point out the principal differences between these two varieties of English.
In British English the present perfect is used to express an action that has occurred in the recent past that has an effect on the present moment.
I lost my key. Can you help me look for it?
In British English the above would be considered incorrect. However, both forms are generally accepted in standard American English. Other differences involving the use of the present perfect in British English and simple past in American English include already, just and yet.
I've just had lunch.
I've already seen that film.
Have you finished your homework yet?
I just had lunch OR I've just had lunch
I've already seen that film OR I already saw that film.
Have you finished your homework yet? OR Did you finish your homework yet?
There are two forms to express possession in English. Have or Have got
Do you have a car?
Have you got a car?
He hasn't got any friends.
He doesn't have any friends.
She has a beautiful new home.
She's got a beautiful new home.
While both forms are correct (and accepted in both British and American English), have got (have you got, he hasn't got, etc.) is generally the preferred form in British English while most speakers of American English employ the have (do you have, he doesn't have etc.)
The past participle of the verb get is gotten in American English.
美式英语：He's gotten much better at playing tennis.（他网球打的好多了。）
英式英语：He's got much better at playing tennis.
'Have got' is used predominately in British English to indicated 'have' in the sense of possession. Strangely, this form is also used in the United States with the British participle 'got', rather than 'gotten'! Americans will also use 'have got to' in the sense of 'have to' for responsibilities.
The largest differences between British and American English lie in the choice of vocabulary. Some words mean different things in the two varieties for example:
Mean: (American English - angry, bad humored, British English - not generous, tight fisted)
There are many more examples (too many for me to list here). If there is a difference in usage, your dictionary will note the different meanings in its definition of the term. Many vocabulary items are also used in one form and not in the other. One of the best examples of this is the terminology used for automobiles.
Once again, your dictionary should list whether the term is used in British English or American English.
For a more complete list of the vocabulary differences between British and American English use this British vs. American English vocabulary tool.
Here are some general differences between British and American spellings:
Words ending in -or (American) -our (British) color, colour, humor, humour, flavor, flavour etc.
Words ending in -ize (American) -ise (British) recognize, recognise, patronize, patronise etc.
以 -or（美式）-our（英式）结尾的单词：color, colour, humor, humour, flavor, flavour 等等。
以-ize（美式）-ise（英式）结尾的单词：recognize, recognise, patronize, patronise 等等。
The best way to make sure that you are being consistent in your spelling is to use the spell check on your word processor (if you are using the computer of course) and choose which variety of English you would like. As you can see, there are really very few differences between standard British English and standard American English. However, the largest difference is probably that of the choice of vocabulary and pronunciation.