Yum is a designed to help you maintain a system relative to a well-maintained rpm repository. In order for this to work, it is important to clean up your system so that its RPM database is consistent with what is actually installed on your system. Installing yum on a brand new, clean system installed from RPMs is ideal. If your system has been in use for a while, though, don't panic. Yum may install and work just fine.
However, if you have ever used the -force and/or -nodeps options to install RPM packages, or if you've built and installed a number of programs (perhaps into /usr/local) from source, you may encounter problems. In these cases the best thing to do is to do your best to remove the offending packages or programs and reinstall them from a consistent set of RPMs built for the base distribution you are using.