I have installed SQL PLUS from instructions I got from hereSQL PLUS .. BUt now I don't know how to start whenever I start it using sqlplus64 command it asks for username and password which I don't get during it's installation. Now I want to know how to start SQL PLUS
Your default users are sys and system. According to the Oracle website the passwords are displayed in a message during installation, but I guess you didn't see it?
It used to be that the default users had default passwords. For sys it was: change_on_install and for system: manager. As you can guess these weren't changed on install, which generally caused a major security risk so maybe this is why Oracle stopped using them.
So my advice would be: - try the above default passwords and hope they work (which is unlikely); - reinstall the database and look for the message giving you passwords.
Once you are logged in as sys, create a day-to-day user and use it for your project. Don't use sys or system - they are too powerful and to be used for administration only.