What did you try?
Take a look here: Version Control with subversion
, especially the chapter about server configuration (chapter 6).
There are a few ways to set up authentication/authorization: I choose for apache/ldap/svn.
Without knowing what you tried (and what worked/failed) it will be a bit hard to help you.
Google (or any other searchengine) can help as well, feed it with the following: svn authorization authentication
BTW: I'm assuming you use svn 1.5.
Hope this helps.