Ambari requires postgresql 8.x
It does not support 9.0 as of today.
You should resolve all the dependencies for 8.x in order to have Ambari installed an run.
In general, if you have all the repositories configured correctly, ambari should resolve all the dependencies automaticaly and install all needed packages.
Please, remove your manually installed postgres 9.0 and run "yum install ambari-server" again.
Post all the error messages here.
Make sure you set repositories correctly before running yum install...