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"man yum.conf" should tell you everything you need to know about how to set-up yum.conf.
You will need to be root to edit /etc/yum.conf. ("su", and it will ask for the root password.) You can edit it using any text editor. I like "kate" for GUI, or "pico" for terminal-based editor (only because I remember Wordstar...)
You can view it with "cat /etc/yum.conf", "less /etc/yum.conf", or "more /etc/yum.conf" if for some reason you don't have "less".
You need to get a good book or online tutorial on basic Linux use an administration. For basic basics, I like the inexpensive "Phrasebook" series of pocket handbooks. (i.e. "Linux Phrasebook", "Apache Phrasebook", "MySQL Phrasebook")