As a matter of fact, I can speak both and that’s why I’ll teach you the two options so that you can choose the dialect that suits you best.
First of all, let’s see some differences in spelling:
It looks like British tend to write words in a more complicated way, doesn’t it? Well, it’s only a reflection of their conservative nature. They conserved the words as they were in the past when they borrowed them, mostly from French. Americans, on the other hand, have helped the language evolve and prefer to write the words as they are pronounced.
However, the main differences between British and American English (and all the other English dialects in the world) concern the vocabulary, especially the most colloquial one. For this reason, we can find mostly different words when we talk about means of transport, house appliances and furniture, food, clothes, sports… even some seasons!
The list is extremely long, so I’ll send you to a very helpful Pinterest page. To put you an example of the different words: a British lorry is an American truck, a bag in the UK is a purse in the US, and a waistcoast is a vest. Also, you should write your zip code in the UK but your post code in the USA; you will meet your friend in a British café or an American diner with couches or, as British would say, sofas; you would play football in the UK or soccer in the USA and you would be playing the same game (or match for British pals, not American guys); you would live in a flat or in a semi-detached in the UK but an apartment or a duplex in the USA; you would use the toilet in the UK but the restrooms in the US; you could eat prawns and chips in the autumn in the UK but shrimps and fries in the fall in the USA…
Some of these words are extremely confusing even for English speakers because the term exists in both terms but it has different meanings. For example, a bag can also mean the classic plastic bag you get at the supermarket in the USA. The American chips are the British crisps but British chips are American fries. Also, if you study in a public school in the UK it is in fact a private expensive school, while in the USA a public school means it is state-funded and therefore free or very cheap.
To finish with the vocabulary, be very careful with swear words: Americans and British don’t use the same words to swear and insult. Although there are a few common terms such as fuck, shit or bastard, they are not always seen so strong in the different countries and there are others which can make people quite angry or simply confused e.g. an American person would never say dickhead or bollocks, they would rather say Moron, douche or fuck. It is generally known that the expression “be pissed” means “drunk” in the USA but “angry” in the UK… so try not to piss off a British or an American guy. You can do that being polite, so remember to say often pardon or excuse me in the UK and sorry or excuse me in the USA.
Last but not least, everybody knows that Americans and British don’t pronounce the same. Some classic rules are omitting the intermediate “t” in words like “twenty” or “ninety” or making it a rolled “r” like in “better” or “native”. This link may be helpful.
So now that you know the tricks to differentiate British and American English, which one do you prefer?