There are a ton of differences between British English and American English, but I thought I’d highlight some of the ones that stick out in my head that I notice that you may run into.
theater/theatre (er is switched to re)
favorite/favourite (0 is changed to ou in many words)
memorize/memorise (lots of words where the z and s are changed)
check/cheque (ck -> que)
Those are just a few of the differences that I come across often, to see more differences, refer to these websites: