You are a little bit nervous because you do not like speaking in public. You would rather just listen, but your teacher has explained that everyone in the group should contribute to the discussion.

Choose the best way to be a good participant.

Come in strong. Be assertive and aggressive so that you speak before anyone else.
Decide on your own agenda for the discussion. You will know exactly what to say.
Decide what about the topic is meaningful to you. Take notes and find a good time to speak up.
Listen to all the participants speaking. Add a comment only if you have the same ideas.