Hi!
There are differences, but they are mostly quite small and nothing to worry about. I could write a lot about this, but here are the essentials:

• Formal written British and American English are almost identical. There are some small differences in spoken language.
• There are many Englishes in different countries (British, American, Caribbean, Indian, Australian, South African etc.), but there are also different Englishes within countries. In Britain, there are differences between the English spoken in the East Midlands, Scotland, the West Country etc. Sometimes the language differences within a country are bigger than the differences between different countries!
• In general, there are very small grammatical differences between different Englishes.
• There are often some differences in vocabulary, but these are usually quite small, perhaps a few dozen words. Often people will understand different forms of English, even if they only use one themselves. For example, Americans say 'parking lot' and British people say 'car park', but they will understand the other way of describing it, so there is no problem with communication.
• The biggest differences tend to be accents and even native speakers can have problems with a strong accent they are not familiar with.
• There are also some minor differences in punctuation and the way of writing numbers.

Here are some pages on LearnEnglish that can help you with this area. An exercise on British and American English. A video about some of the differences between British and American English. I hope that helps!
Adam
The LearnEnglish Team