To enable Comments, go to the
Settings | Comments tab:

Then select the
Show radio button and click
Save Settings:

Once Comments are enabled, several more settings appear:
Who Can Comment?
Only Registered Users
: if not logged in already, visitors will be prompted to log into Blogger before leaving a comment. If they don't have an account, they can create one.
Anyone
: anybody in the whole wide world can leave comments
Only Members of this Blog
: only members of the blog (if it's a Team Blog) can leave comments
Default for Posts
New Posts Have Comments
: by default, all new posts have Comments enabled
New Posts Do Not Have Comments
: by default, Comments are disabled on new posts, but can be enabled on a by-post basis:
Comments Timestamp Format
- The date and time that comments are left will be displayed in the format you choose here.