Hello
A standard SAP landscape consists of a development system, a test system and the production system.
To get the hardware requirements you can make use of the SAP sizing tool.
http://service.sap.com/quicksizing
Also refer to the below links for more information.
85524 - Sizing SAP Business Solutions (e.g. with the Quick Sizer)
Quick Sizer - Getting started | SCN
The choice of the database and OS depends on the company.
Regards
RB