Please read the documentation at either your downloaded copy of OpenACS (doc/openacs) or at http://openacs.org/doc/openacs.
The Installation Guide has answers for all the questions you posted here (including requirements. Yes, we require PostgreSQL. If you want to use Oracle you can download ACS from www.arsdigita.com. If you want to use another (open source) database you can coordinate with other people that want the same).