Apache supports CGI and PHP by default, so you don't need to configure anything for these. You do need to make sure that you have the php packages installed, but I think that the basic PHP package installed by default.
MySQL will not start until you have set a root password (for it's administration account, which is called 'root'). MySQL usernames are in the form username@hostname, so root@localhost and firstname.lastname@example.org
are separate accounts. This can be difficult to understand and causes problems if you make a mistake at the start, so take a look at the documentation on managing accounts at mysql.com before you go ahead. You don't need to configure anything else to make MySQL work.
/var/www/html is a standard directory. You can access it directly using the system 'root' account, but it is best to enable access through WebDAV or FTP services, which can enable access by username/password.
Red Hat provide a very good set of documentation, which you can download for free from their site (RHN has an ISO file for a Documentation CD as well). The sections on Apache will cover the essentials. Your Apache server includes reference documentation as well - try http://localhost/manual
once you have started the server.
Please take your time and read the documentation before working on Apache and MySQL - unfortunately Linux has no safety mechanisms to prevent you from accidently wrecking the security whilst trying to make things work.