On trac's homepage they have a few links for the documentation that will help you get started as well as getting into some more advanced features.
I have trac installed on a virtual machine, and manage several projects from the same installation.
You must get acquainted with trac's command line tool that allows the initial setup and configuration for each project.

You start by creating an svn repository for each project.
My installation is at /servers/work/...

sudo svnadmin create /servers/work/svn/repo/<PROJECT>
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Having created the repository, you can now initialize your trac environment for the PROJECT

sudo trac-admin /servers/work/trac/env/<PROJECT> initenv
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Also, typing help on trac-admin's prompt will display a list of available commands and how you can set different levels of permissions.

Do read the TracGuide. They have good documentation.

I have trac online and work with the permissions in order to grant access to different kinds of information.
Everyone can see the wiki and the ticket reports, but wiki and ticket creation are granted only to the developers and 'project owners'.