A Few Things to Be Aware of When You Come to the States
The most important thing to realize upon entering the States is that the language that you studied is not the language we speak. In addition to the fact that students in the Czech Republic learn British English, you also focus on very formal English. In the US, our English is rather informal. You will hear things that you were taught were not done. In some dialects of American English, the double negative ("I haven't never," "I don't have no") is not only acceptable, but the preferred form. The best way to handle the difference between what you were taught and what is going on around you is to keep an open mind and take note of how the people around you use their language.
Americans are also known for being bad at Geography. Americans may not know which country you are from, or may even refer to your country by a name it no longer uses. Don't be offended when you run into an American who does not know that there is no such place as Czechoslovakia anymore. That is not to say that there aren't Americans who know more about your country than you do, but they are a little more rare. Be patient and be prepared to talk about your country. For the most part, we are very curious. We just don't get many opportunities to learn first hand about many countries.
Perhaps what I hear the most complaints about, though, is the "fake friendliness" that a lot of first time visitors get confused by. You may be walking and someone you've seen a few times, if that, will ask you, in passing "How are you?". The correct answer to this question is always "Good, you?", no matter how your day has been. Instead of thinking of this as being asked about how you feel, treat this as a form of "Hello." If you're having a bad day, and you respond with "Man, my day has been terrible! I failed a test and realized that I left my lunch at home," all you will do is frustrate the other person. While this situation is more common in the Southern states, this does happen elsewhere, so be prepared.
If you are prepared for these when you arrive in Atlanta or Chicago or Los Angeles, the worst confusion about coming to the states will be over. Good luck!