Once you've created your blog and begun posting regularly, it's nice to allow visitors to leave comments on your posts. Mutual commenting provides writers with valuable feedback and helps build community.
If you're using Blogger, take a tour of your settings. It's a tab in your Dashboard next to the one you use to create new posts. The subsection for managing comments is shown in the screenshot below (click to see a larger version):
Many of the relevant settings are pretty self-explanatory, but farther down the page, there are a few that bear explanation.
In "Who Can Comment", users with "google accounts" means that a commenter can identify themselves by their gmail username. "Registered Users" means people who have registered with Blogger - so once they've created an account on your blog, they can use it to identify themselves on any other Blogger blog. This feature also allows commenters to use OpenID and possibly other third-party comment tracking systems. There's a whole world of services out there!
"Comment Moderation" allows you to read people's comments before they become visible to other readers. You might enable this if comment-spam becomes a problem. Of course, that means you have to stay on top of things in order to allow conversation to flow.
"Show Word Verification" will force commenters to type the letters of a CAPTCHA before the comment will be accepted.
Lastly, here is Google/Blogger's help page on the subject: