American English vs British English what’s the difference ?

There is very little difference.  Accent is the main difference.  There's a minor difference in spelling in a handful or words, such as "color" versus "colour", "program" versus "programme", "tire" versus "tyre".  

In the US a public toilet is a "restroom", whereas in Britain it's called "the loo".

In the US cars have a "trunk", but in Britain they call it "the boot". 

That's pretty much it. If you can speak standard English, you can understand it in any English-speaking country on the planet. It is remarkably uniform except for those few differences.

Also it's more correct to say "North American" English, because the speech in Canada is virtually identical to the US.

To "Anonymous":  no, in the US, a "modest dress" means it's not fancy, not expensive-looking.  If a PERSON is described as modest, it means they wouldn't show a lot of skin.   It has nothing to do with the dress, it has to do with the PERSON. You really should refrain from making such generalized claims.

And the proper way to write "Hey ho" as a  greeting is "Hey-ho".

Less than 1% difference. 

Now that accent... 

More than 1% difference! 

Lol!!

Mainly just some vocabulary and a limited number of spellings. The grammar of the standard language is the same.  Slang differs, of course.

The differences are with the accent, spellings of some words and slang. Both can still be understood by each other though, since they are basically the same aside from those points :D

Quite a lot, but in reality both versions have dialects that overlap.

The first thing learners notice is different spelling conventions.  The ones that stand out are the -our vs -or spellings, and -ize vs -ise (even though the Oxford English Dictionary prefers -ize!), tire vs tyre, theater vs theatre, traveler vs traveller, jewelry vs jewellery, program vs programme, burglarize vs burgle, utilize vs use, etc.

American English has preserved more strong verbs, so use "have gotten" where standard British uses "have got" (but some dialects do use gotten because they never lost it).

We form phrasal nouns differently, drink driving vs drunk driving, driving licence vs driver's license (note spelling).

Words can have different subtexts even when they mean the same thing on the surface.  When Harry and Meghan got married the commentators described Meghan's dress as "modest."  British listeners would have understood this as "not flashy" in the context whilst/ while American listeners would have understood it to mean "not showing a lot of skin."  

The list goes on and on.  Sometimes people get into real clashes online because they misunderstand each other and have no idea that some word or three along the way mean different things and so the other person has gone down the wrong thought track.  There was a funny twitter tizzy a few years ago where a Brit had commented "hey ho" on an American's tweet.  This is roughly the equivalent of a shrug or a only a little sarky "diddums," which was entirely appropriate to the tweet, but the American who read it understood it as "Hey, whore" and got all her followers to attacked the poor tweeter who had no idea what she'd done wrong.  It was funny to watch but it must've been awful for the tweeter who was chased off the site, at least for a while.

Spelling is a common difference. We use the letter 'z' in many words in which you'll see the letter 's' used. An example is "civilized" rather than "civilised."

There are also many colloquial terms that are used in each.

We will form a "line", whereas they will "queue."