Don't wear a Rangers shirt to a Flyers game in Philly...and vice versa.
Oh, and we hate futbol (soccer) so don't bring it up in a conversation...we don't give a hoot about Manchester City or Manchester United or any other Chesters.
I am American. People in NYC are not tough, we just don't like bs. People in LA though are mean. People in the midwest are super friendly. People in the South are very funny and polite. Some people are nice, some mean, some smart, some dumb, etc.
The culture takes a lifetime to learn. I recommend visiting a national park, a historic center, a mall, etc lol.