Drupal as an alternative for my CMS
Hi.
For more than 5 years I have been developing using a CMS created by me. I'm thinking about migrating to Drupal but then again I have very specific needs. For instance, I have these requirements.
- I have a site which will have 3 sets of menus. The upper menu, the middle menu and the side menu.
- The site will be divided in 7 mini-sites (E.g/ main, students, faculty, administrative, etc)
- The top menu will let you choose between the sites.
- The middle menu will let you navigate the site you’re in.
- The side menu is the secondary menu for the sections of the middle menu.
Also,
- All content must be moderated by the communications department (no content must be published with their revision).
Is Drupal for me? ¿Where I can find info on how to start the multi-site platform?

Welcome to Drupal
Yes, you can do all of those things (and more!) in Drupal.
There are a lot of different ways to handle the menus. I'd suggest searching around a bit in the forums for "second level menu" and "third level menu" to find some of the different approaches that people have taken to solving that kind of problem.
Exactly how you want to divide the site into multiple sections depends in part on how different you want those sections to be in terms of URL, layout, content, functionality, user accounts, etc. If it's possible, a link to your existing site would probably help people decide what to recommend. My hunch is that "multi-site" (which is a technical term in Drupal for independent sites that share a common codebase and sometimes database) is not what you want, and that you might be better off using the Organic Groups module.
For moderation purposes, I'd take a look at the Workflow module.