Look at the end of a published post. There is a spot that says "comments". Click on that to make a comment about the post. That is how your students respond, but they have to be signed in to their own blogs to be able to leave a comment.
That is also where you respond to their blog posts. If you need some examples, go back to my classroom blog and look at the post about leaving advice. (There is a link for my blog over to the right under "Other Links." It says "Sims' Blog".) When you get there, look at the end of that post, and click on "41 comments". A separate window will pop up and if you wanted, you could read everything all the students wrote.
You will also want to "Follow" all your students' blogs. By doing that, every time they do something on their blog, Blogger will notify you on your "dashboard" on the "Reading List".
Don't forget to E me if you have any questions or need more help. I'd also be willing to come back one morning if there are some of you who would like me to spend some more time with you. (No charge! I'm a cheap date!) Happy blogging! I think that the more you use it, the more you'll love it!