There are a acouple of ways of adding repositories in Fedora/Red Hat. All repositories are stored in the
/etc/yum.repos.d directory in Fedora/Redhat.
For well known repositories it is best and more simpler to install a .rpm package to add the repositories to your system. Such as below, which will add the livna-release repositories to your system. You would substitute the URL with one you have found on the internet.
rpm -Uvh http://ftp-stud.fht-esslingen.de/pub/Mirrors/rpm.livna.org/livna-release.rpm
For repositories that need to be installed manually it is a little bit different we have to create a