So don't sit glum and silent, frozen in fear of appearing foolish. Keep alert to the thread of the discussion, and make sure you join in more than once. To confess ignorance is the beginning of wisdom. Try to build a discussion around five or six main points. Talk from brief notes, or even simple headings.
Most novices prepare too much material.
Most novice speakers are either too fast or too quiet - or both. Seminar rooms usually have a chalkboard or whiteboard. You should write up unfamiliar names and perhaps important references.