There are quite a lot of differences between American and British English, so a basic task is probably to identify some categories into which the differences can be placed.  For example, there are spelling differences (e.g., color versus colour; traveling versus travelling), differences is names for things (trunk of a car versus boot of a car; sidewalk versus pavement or footpath), and a variety of idioms (sorry, can't think of any at the moment - but I've lived in both countries and have noticed them).

I don't know whether it would be of interest to you, but there are indeterminate differences, too.  For example, about half of English people apparently spell words like organization as do the Americans (i.e., with a z), but the other half replace the z with an s - as do Australians and New Zealanders.

There are also differences in pronunciation, too, with Americans more likely to exhibit rhotic speech (sounding their r's more noticeably) than do most English people.

There are a number of sites on the web that should help you.

Robert.

Anotehr difference is in grammar, particularly collective nouns.

In America we say "Congress has..." done something, but in the UK, they say "Parliment have..." done something.

Thank you very much, sir Robert Trevethan

 Thank you very much, sir Michael W. Marek

Some words have different meanings depending on whether they are used in anAmerican English or a British English context. The word 'pants' in American English, for example, refers to an item of clothing which is used to cover the legs (i.e. trousers), whereas in British English the word refers to underwear.