Hey there! I hope you have lots of fun in the States. Life-long American scum here to dispel a couple things so you can better enjoy your trip.
For starters, despite all the minor lingo differences, Americans and British are really not as different as you might think. Many of my pals and coworkers over the years who have hailed from the UK have been surprised at how quickly they were able to slip into social norms. You may have some fun conversations at first about the origin of your accent, I suggest you play it up - Americans ladies are into it!
Secondly, remember that everyone's views of other countries are colored by the media's display. So if you're coming here expecting knife fights, cowboy hats, street wars and AK-47s, prepare to be disappointed. It's really safe here, I promise.
As mentioned by others, the size of the country is massive and there are lots of different kinds of Americans! Depending on where you travel to, your experience will be completely different compared to any future trip you decide to take. Enjoy your time! Don't focus too much on the little stuff and if you end up in Texas, make sure you get some barbeque. You'll never look at meat the same way again.