If you are using fedora core, then you use the yum.conf suggested in www.fedorafaq.org
For others, you need to check the distro faq. There will be official repos for distributions which have yum bundled.
Some app maintainers provide yum and/or apt support too - check their faq etc. Usually they make a bit of a thing about it.
Whatever you do, first go look in the menue on the right, called "My LQ" >>>--->
Click on "Edit Profile" and scroll through and set your distribution type and some sort of (informative) location.