I used to be a heavy lurker, too. I still don;t post as often as some of our more prolific members. But seriously, don't be scared. Posting here can be very rewarding.
It's actually great when someone can prove you wrong - you learn a
lot, about debating, about the way you think, about what your beliefs are based on, and of course about the topic itself. The experience is different from just reading a book or lurking.
Don't worry about looking foolish or feeling overwhelmed. It's okay to ask questions. It's
okay to be wrong, even. We don't exactly bite, and there's a lot to gain. I give partial credit to EvC for some of the most significant changes in my life - I approach the world differently after coming here (and a few other sites, including youarenotsosmart.com, LessWrong.com, CreationTheory.org, and others), and the changes were all extremely positive.