The user will have to register (if you enable self-registration in the settings) or you'll have to create an account for them. Then the admin has to create a blog and give the new user permission to post on it.

But how i creat a bolg and give premission, i can't do it....:(

Are you the admin? Did you install the blog? If you have permission to create a blog, then go to the Blogs tab and click on 'New blog'.