First of all, each term is significantly shorter in the US. For the semester system, each semester is less than 4 months. For the quarter system, each quarter is less than 3 months. 

Graduate schools in the US are very serious. The pace can be very fast. You may have quite a lot of homework and some time-consuming projects. You really need to work hard from day one. 

The best grade is A, instead of 100. Therefore, you don't need to be perfect to get the highest grade. Besides the final exam, certain percentage of your grade will come from homework, mid-term exam, projects, papers, and class participation. Gruduate seminars usually don't have a final exam. You typically need to read papers, do projects, and write a paper. Read the papers early and see if you have some new ideas. Talk to the professor to evaluate these ideas. At the end, an idea should be transformed to a project and/or a paper,

Here students are welcome to ask questions in class. No question is thought as stupid. Asking questions is viewed as active class participation. Giving the professor a good impression is as important as getting a good score in the exam. If you never ask a question or talk to the professor, he will have no chance of knowing your talent. If it's hard for you to communicate verbally, start with e-mails.