first, logging in as root: in an xterm/console/konsole/gnome-terminal ( whatever you have handy) do:
passwd being whatever you chose during the installation for the superuser/administrator passwd.
for mplayer, look at:
for instructions as well as what you'll need.
for c or c++ compilers, type in your console:
and see if you get any output. if you get "command not found" then you don't have a c compiler installed and will have to install one ( log in as root and run rpmdrake using your install cd's for the source, install everything development related).
for c++, g++ is the compiler (gcc for c).
you will need more than a compiler to compile applications ( you'll need most of the development tools your distro provides, like autoconf, automake, make, m4, perl, binutils, libtool, gcc, g++, most rpm distros I believe also split libstdc++ off into a seperate package - you'll need that one for c++ code, and so on).