If you mean that you want to make the same post in multiple sites at the same time, there is not a way in multisite to do this. All the sites are separate.
There are other ways to to this (http://premium.wpmudev.org/blog/how-to-publish-to-multiple-wordpress-sites-from-a-single-install/, and through a service like Manage WP),
BUT…
You should know that publishing duplicated content will tank your search engine rankings for those sites, so you might want to think twice about doing it.
There are plugins that do it too, called broadcasting plugins, but what Bet said is true. SEO gets shot when you do this.
