What is the recommended OS for VPS? most searches show CENTOS or Debian.
Both CentOS and Debian are excellent operating systems, if you choose either of them, you won't go wrong. Debian is certainly rocksolid OS with great security track-records. However, Since you are new with Linux machines, you would need some sort of control panel to manage the Linux machine.Hence, I would recommend CentOS box with cPanel for easy startup.
What platform is recommended? 64bit / 32bit ?
64 Bit Operating system better Of course.
what are the basic installation necessary for an operational VPS (web server) ? What are the extra recommended installation?
cPanel will include all necessary tools and software to run web server. However, if you wish you can install firewall or other required tools through terminal. Also, you can try out CentOS in virtual box easily, for cPanel, you would need license and static IP address.