In many classes, no matter what the dicipline or size of the class, there is an expectancy that you participate. The professor may even make participation in class a requirement, worth up to 15 or even 20 percent of your class grade. For that reason it is important to know how to participate in class. Don't be shy.
Participation takes practice, and there is no time like the present to start.
You do not have to agree with the professor to get a good grade. In fact, students who disagree with the professor and put thought into their arguements, are likely to get more out of the class, than those that agree with the professor without questioning.