Re: American English x British English
They are not very different in practise. We British understand what Americans say and write, and they understand what we say and write. There are some differences in spelling, some differences in phraseology, and some differences in the meaning of common words. If you Google something like "British English versus American English", you will find all the details you need about the main differences.
I'm not a teacher of English, but I have spoken it for (almost) all of my life....