Multi user in wordpress, how to do it

You can assign registered subscribers as authors. Otherwise you can make the default role author. Also user roles can be modified for specific access on the back end. You can do that with a plugin like Role Scoper.

If you want it so each author only see’s their own posts and media, you could try this code posted by a member named pcserveis in this thread.

It goes in functions.php. I pasted it in right after the first php tag, it worked.

Just a security tip; setting each new registered user to author is very dangerous. Malicious users might notice this and post malware related links on your website. I would recommend you to set each new user as subscriber, and change them to author manually, kind of quarantining them and approving them manually.

Agreed. Another user-freindly method (and good way to keep users out of admin) is to use a front end post form, and moderate posts before publishing.