As splitimage pointed out, there's usually a script for chairing meetings, so I wouldn't worry about opening or closing the meetings. If you're worried about choosing a topic I've always found the book "As Bill Sees It" is good for finding topics. On each page there's a brief reading that suggests a topic. This takes the onus off of you. I know I was nervous the first time I chaired a meeting, so don't feel alone. Once you get through your first meeting it'll be a piece of cake. Good luck.