I plan to use Literature Circles this year too. First I'm going to go over some keys to a good discussion with my students. General expectations of listening carefully, looking at the speaker, using text ideas to support
comments etc.
Next I will read a book to the entire class and model the first role of Discussion Director with the entire class. The next day I'd read another book and have all students break into groups and practice that role.
The process can be involved but there is structure you can develop. Check out...
www.lauracandler.com She has so much information on literature circles
with forms you may find helpful as guides for your students.
Consider reading Literature Circles by Harvey Daniels. He also has a website.