[Please note that this post does NOT have "For Credit" in the title--that means this one is just information, so anything you post in response will not count toward the blogging requirement.]
It may take you a few posts before you get comfortable with writing for the blog. The best way is just to jump in! One advantage of the blog is that it's fairly fast-paced and emphemeral. If you post something that you later realize was ill-informed, or poorly expressed, or just plain dumb, it's quickly forgotten and you'll have plenty of opportunities to put a better foot forward. That's part of the point: the blog exists so that you speculate, think-out-loud, and take some interpretive risks, without the pressure of classroom discussion or the high stakes of a paper assignment.
Blog posts don't need to be long. Just because the question is one you COULD write a three-page paper about, that doesn't mean that you have to! In fact, the blog works best if responses are short and to the point. A few (2 - 5 sentences) are plenty to stake a specific claim and back it up (a direct quote from the text helps!), while leaving plenty for your classmates to say.
It's also okay to disagree: with a classmate, with the general direction of the responses, with the way the question was framed in the first place. The blog is there to help you learn and make better sense of the novels we read. If a particular blog conversation is frustrating you because it seems off-point or not to be going anywhere, or to skirt issues you find more compelling, you can say so!--particularly if your opinion is accompanied with a suggestion for an alternative way to think about the text or issue in question.
And it's fine to build on someone else's point. "Yes, I agree!" doesn't contribute much to the conversation, but developing someone else's idea or taking it in a different direction can be helpful: e.g., "what Pat said about XXX also explains YYY" or "I think Pat's point could be taken even further..." or "I think Pat is right but a different episode explains the point better."