Seminar format may vary from professor to professor, but most involve a series of assigned readings that are presented in class for discussion, often by a student discussion leader. Some seminars require papers that may also be presented in class, written reviews of reading materials, or some other form of written work. Take notes while you read. Jot down questions you have about the readings. Formulate and write down 3-4 questions that might be good for discussion.
Some people are nervous about speaking in front of other people.
For a seminar, that can pose problems, because a seminar is about the free exchange of ideas. For it to be a success, people have to talk. You have to feel comfortable enough to forget your worries. Think of the seminar, including the professor, as a group of friends having a friendly chat about some interesting ideas. Pay attention during the presentations and discussions.