Ok
Well, open a terminal (like an xterm) and log as root (just need to type "su" and it will prompt you for a password).
Then type the 2 commands. If that works you can add them permanently. For that add your gateway in /etc/gateway file. Add your DNS in /etc/resolv.conf. To configure permanently eth0...it's in /etc/sysconfig... I don't remember... but do:
cd /etc
find . -name \*eth0\*
and it's a file called ifup-eth0 or something similar