For most of the OpenACS modules, you can configure them through the admin pages, accessible through /admin from any browser. You must be registered as a site-wide administrator to access those pages (refer to the installation guide to know how to do that).
By visiting the /admin pages, you'll have most of the modules in front of you, and you'll have the opportunity to set them up. This is a great way to explore the possibilities that OpenACS opens to you.
If you see a module that you don't understand how to setup, just visit the /doc directory to learn how that module works and what it's intended to do.
For the ecommerce module, type in /admin/ecommerce to be taken to its admin pages.
URL: /admin/content-sections
The content sections module allows you to add a "section" of the website, static or dynamic, to the users' "Workspace" and tell if that's publicly available or only to registered users, plus provide an introduction and help to the users.
URL: /admin/ug
User groups are a great feature of OpenACS and gives you lots of flexibility. Unfortunately, as of this release, not all modules are user groups-ready, but that should change with OpenACS 4 (coming this summer).
There are User Group Types and User Groups. One user group type can have several user groups, which in turn can have several subgroups. Each user group can have modules associated with it, that members of that group will be able to use and that will be specific to that group.
For example, in the USU Free Software and GNU/Linux Club we had several projects that were being carried by club members and we wanted to provide a way for them to collaborate. Something besides mailing lists. So we created a group type called projects and several groups inside it, one for each project. Each group had an administrator assigned to it, who had control over that group and that could create subgroups if he/she thought necessary. OpenACS provides group pages, all ready, in /groups.
There's much power in user groups and I highly recommed you to read the documentation about it on /doc/ug.