Start with a class in which you feel comfortable
Make your first efforts to participate in class that feels comfortable. Perhaps you will choose a class with fewer students; or a class being taught by an instructor who seems particularly supportive; or a class where you sit beside a good friend.
Be prepared for class
The people who feel most comfortable speaking in class are often the ones who have done the assigned readings and have thought about the topic in advance. Before class, read, write, think and talk to others. If it helps, write your questions and ideas down on a piece of paper and refer to it in class. By using these strategies, you will feel more confident that you understand the material and you will therefore be more prepared to talk about it.
Sit in front of the class
If looking out over a sea of faces makes you uncomfortable, try sitting in the front of the classroom. Then, when you speak, imagine you are simply talking to the instructor and a few students.