Create a new revision automatically on edit
To enabled automatic revision creation go to the content type page (admin/content/node-type) and select edit for whatever content type you wish to have revision created automatically.

Under the workflow fieldset there's going to be an option "Create new revision" make sure you check that box and save your content type settings.

Now under your permissions (admin/user/permissions) be sure to not to give your users the "administer nodes" permission. This will prevent them from overwriting that option.